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.

Reviewed by: dim
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11314