tests: Consolidate Linux helper build flags and brand binaries

Define the compiler and linker flags used for static Linux test
helpers in one place and use a common rule to build all helpers.

Explicitly brand the resulting binaries as Linux. Clang and lld
otherwise produce static ELF binaries with the System V OSABI and no
ABI note, which only worked when kern.elf64.fallback_brand was set to
Linux.

This makes the tests independent of the host's fallback ELF brand
configuration and simplifies adding more Linux helpers.

tests: Refine Linuxulator test infrastructure

Move the Linuxulator userland tests from sys/tests/linux to
tests/sys/linux. sys/tests is intended for kernel-side test modules,
while ATF tests and their helper programs belong under tests/sys.

Use the generated architecture-specific Linux syscall numbers instead
of embedding raw syscall numbers in the test support code. Extend the
low-level syscall primitive to pass all six Linux ABI arguments on
amd64 and arm64.

Use close_range(2) to close the standard descriptors in the devfs PTY
helper, remove the unnecessary crt0 header, and use the FreeBSD
amd64 spelling for the architecture check.

Only descend into tests/sys/linux on architectures for which the
freestanding Linux helpers can currently be built.

Suggested by: ngie
Differential Revision: https://reviews.freebsd.org/D60702

tests: Check native controlling terminal release on last close

Add a regression test verifying the native FreeBSD controlling terminal
semantics.

Create a new session, acquire a pseudo-terminal slave as the controlling
terminal, and close the last descriptor referencing the slave. Verify
that a subsequent open of /dev/tty fails with ENXIO.

Use pipes to synchronize the parent and child so that the tested close is
guaranteed to be the final slave close.

This complements the Linuxulator test, where the controlling terminal is
expected to persist across the last close.

tests: Add Linux epoll descriptor inheritance regression test

Add a Linux ABI regression test for preserving epoll descriptors across
fork. This covers the behavior implemented by 226616ad7dba ("linux:
preserve epoll descriptors across fork using KQUEUE_CPONFORK").

Build the test subject as a static, freestanding Linux ELF executable,
using the common Linux test runtime. A native ATF wrapper executes the
helper under Linuxulator and reports its exit status through Kyua. No
Linux base system or Linux libc is required.

Exercise epoll instances created both with and without EPOLL_CLOEXEC.
For each variant, verify in the child process that:

• the epoll descriptor remains valid after fork;
• F_GETFD reports the expected FD_CLOEXEC state;
• an ordinary pipe descriptor is inherited as a control case;
• a ready registration and its event data are available;
• epoll_ctl(EPOLL_CTL_MOD) succeeds after allocating additional file descriptors.

The final check reproduces the descriptor-reuse failure mode seen before
the fix. When the epoll descriptor was omitted from the copied file
descriptor table, a newly created pipe could reuse its number, causing
subsequent epoll_ctl calls to fail with EINVAL.

Limit the test to the semantics provided by KQUEUE_CPONFORK. It does not
test shared epoll state between parent and child: registration changes
and one-shot consumption are still independent because the underlying
kqueue is copied rather than shared.

Extend the reusable freestanding Linux syscall layer with the fork,
wait4, pipe, fcntl, and epoll operations needed by this test. Provide
the appropriate syscall implementations for Linux x86-64 and AArch64.

The test passes on FreeBSD 16-CURRENT arm64 with linux64.ko.

tests: Generate Linux syscall stubs for Linuxulator tests

Generate freestanding Linux syscall stubs directly from the Linuxulator
syscalls.master files instead of maintaining architecture-specific syscall
number mappings and C wrappers.

Emit stubs for all native syscalls and place each one in a separate ELF
section. Link the test helpers with --gc-sections so that only referenced
stubs are retained.

Use syscalls available on both amd64 and arm64 directly, including openat,
pipe2, clone, epoll_pwait, wait4, and exit_group. This removes the
architecture-specific compatibility wrappers and linux_syscalls.c.

Give the generated stubs libc-like error handling: translate raw Linux
error returns to -1 and store the positive error number in errno. The
test helpers are single-threaded, so a non-TLS errno is sufficient.

Use the existing Linuxulator event and errno definitions where their
headers are self-contained, and retain local definitions only for ABI
constants whose kernel headers cannot be included by freestanding
userspace code.

Tested on FreeBSD/aarch64 with:

make -C /usr/src/tests/sys/linux clean all check

Both devfs_pty and epoll_fork pass. The Linux helper binaries also
cross-build successfully for amd64.