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.

Tested by: swills
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11424