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

[emaste]

Summary

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 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.

Test Plan

nuc% LD=false make -V LINKER_TYPE -V LINKER_VERSION
make: "/usr/home/emaste/src/freebsd-wip/share/mk/bsd.linker.mk" line 47: warning: Unable to determine linker type from LD=false
make: "/usr/home/emaste/src/freebsd-wip/share/mk/bsd.linker.mk" line 56: warning: Unknown linker from LD=false: none, defaulting to bfd
bfd
21750

aside, the line numbers reported by bmake are incorrect

[emaste]

share/mk/bsd.linker.mk

50 ↗

(On Diff #30253)

I will also remove the 2>/dev/null, from my local commit:

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.

nuc% LD="ld -mbogus" make -V LINKER_TYPE -V LINKER_VERSION
ld: unrecognised emulation mode: bogus
Supported emulations: elf_x86_64_fbsd elf_i386_fbsd
make: "/usr/home/emaste/src/freebsd-wip/share/mk/bsd.linker.mk" line 47: warning: Unable to determine linker type from LD=ld -mbogus
make: "/usr/home/emaste/src/freebsd-wip/share/mk/bsd.linker.mk" line 56: warning: Unknown linker from LD=ld -mbogus: none, defaulting to bfd
bfd
21750

[swills]

Confirming this fixes the misp64 build for me.

[emaste]

Oops, should have had Submitted by: bdrewery. He's afk for a bit, but before that we discussed committing something like this in case of continuing problems.