MFC kernel build-id support

MFC r320244, r320245, r320246, r320247, r320248, r320249,

r320250 (bdrewery):

Add basic bsd.linker.mk auto included from bsd.compiler.mk.
This will provide LINKER_TYPE and LINKER_VERSION.

Support XLD for setting X_LINKER_TYPE and X_LINKER_VERSION.
This is similar to r300350 for bsd.compiler.mk.

Support cached linker values in environment.
This is similar to r289659 for bsd.compiler.mk.

LINKER_VERSION: Support external binutils.
The ports binutils stores the version in the 5th word so just look for
a version using a pattern instead.

Don't overwrite already-set LINKER_VERSION/LINKER_TYPE.

Provide proper values for X_LINKER_TYPE/VERSION when XLD == LD.

Pass along LINKER_* vars during installworld and show in test-system-compiler.

MFC r320258, r320272, r320275, r320502 (emaste):

change GNU ld LINKER_TYPE from binutils to bfd

GNU binutils includes two linkers: ld.bfd and ld.gold. For clarity use
LINKER_TYPE=bfd to refer to ld.bfd, the original binutils linker that
identifies itself as "GNU ld".

bsd.linker.mk: add band-aid for linker invocation failure

In some cases bsd.linker.mk reports an error like:

make[4]: ".../share/mk/bsd.linker.mk" line 56:
Unknown linker from LD=ld -m elf32ppc_fbsd:"

For now change this to a .warning, and then assume GNU ld 2.17.50.
At present the linker type detection is used only for enabling build-id,
and we can carry on without it when type detection fails.

Also, show errors from ${LD} --version to aid in failure diagnosis.
Successful invocations of ${LD} --version produce no output on stderr
so this will not create any spam in non-failing builds.

enable --build-id for the kernel link

A Build-ID is an identifier generated at link time to uniquely identify
ELF binaries. It allows efficient confirmation that an executable or
shared library and a corresponding standalone debuginfo file match.
(Otherwise, a checksum of the debuginfo file must be calculated when
opening it in a debugger.)

The FreeBSD base system includes GNU bfd ld 2.17.50 as the linker for
architectures other than arm64. Build-ID support was added to bfd ld
shortly after that version, so was not previously available to us.

We can now start making use of Build-ID as we migrate to using lld or
bfd ld from ports, conditionally enabled based on the LINKER_TYPE and
LINKER_VERSION make variables added in r320244 and subsequent commits.

Introduce LINKER_FEATURES to avoid duplicating version logic

MFC r327857 (bdrewery, submitted by Dan McGregor):
Cache LINKER_FEATURES to fix the wrong ones being used.

Sponsored by: Dell EMC Isilon
Sponsored by: The FreeBSD Foundation