MFC r354709-r354710, r355069, r355439, r355780, r356273, r356472

r354709:
Rewrite arm/stack_machdep.c for EABI; add stack(9) support to arm kernels.

The old stack_machdep.c code was written for the APCS ABI (aka "oldabi").
When we switched to ARM EABI (back in freebsd 10) this file never got
updated, and apparently nobody noticed that until now.

The new implementation uses the same stack unwinder code used by the
arm implemenation of the db_trace stuff.

r354710:
Compile in arm/unwind.c if options STACK is in effect; the new arm stack(9)
code now uses unwind.c.

r355069:
When doing ARM stack unwinding as part of stack_save(9), do not search
loaded modules (pass 0/false for the can_lock arg). Searching the unwind
info in modules acquires an exclusive sxlock, and the stack(9) functions can
be called in a context where unbounded sleeps are forbidden (such as from
the witness checkorder code).

Just ignoring the existence of modules in stack_save() is not ideal, so I'm
looking for a better solution, but this commit will make it possible to boot
an ARM kernel with WITNESS enabled again, until I get something better.

PR: 242200

r355439:
Declare the global kernel symbols created by ldscript.arm in arm's machdep.h,
and remove a couple scattered local declarations.

Most of these aren't referenced in C code (there are some references in
asm code), and they also aren't documented anywhere. This helps a bit
with the latter.

r355780:
Rewrite arm kernel stack unwind code to work when unwinding through modules.

The arm kernel stack unwinder has apparently never been able to unwind when
the path of execution leads through a kernel module. There was code that
tried to handle modules by looking for the unwind data in them, but it did
so by trying to find symbols which have never existed in arm kernel
modules. That caused the unwind code to panic, and because part of panic
handling calls into the unwind code, that just created a recursion loop.

Locating the unwind data in a loaded module requires accessing the Elf
section headers to find the SHT_ARM_EXIDX section. For preloaded modules
those headers are present in a metadata blob. For dynamically loaded
modules, the headers are present only while the loading is in progress; the
memory is freed once the module is ready to use. For that reason, there is
new code in kern/link_elf.c, wrapped in #ifdef arm, to extract the
unwind info while the headers are loaded. The values are saved into new
fields in the linker_file structure which are also conditional on arm.

In arm/unwind.c there is new code to locally cache the per-module info
needed to find the unwind tables. The local cache is crafted for lockless
read access, because the unwind code often needs to run in context where
sleeping is not allowed. A large comment block describes the local cache
list, so I won't repeat it all here.

r356273:
Since arm/unwind.c s conditionally compiled, only call functions in it
when one of those conditions is true. Fixes build failure on kernel
configs with no debugging options active.

r356472:
Add #ifdef option-test wrappers around another call to an arm/unwind.c
function which is only compiled-in with certain options.

Why is it always the most trivial part of a big commit that takes 3 tries
to get right?