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

sys/conf/kern.pre.mk

@@ -114,6 +114,10 @@ DEFINED_PROF=	${PROF}
 # can override the others.
 CFLAGS+=	${CONF_CFLAGS}
 
+.if ${LINKER_TYPE} != "bfd" || ${LINKER_VERSION} > 21750
+LDFLAGS+=	-Wl,--build-id=sha1
+.endif
+
 # Optional linting. This can be overridden in /etc/make.conf.
 LINTFLAGS=	${LINTOBJKERNFLAGS}
 

sys/conf/kmod.mk

@@ -125,6 +125,10 @@ CFLAGS.gcc+= --param large-function-growth=1000
 CFLAGS+=	-fno-common
 LDFLAGS+=	-d -warn-common
 
+.if ${LINKER_TYPE} != "bfd" || ${LINKER_VERSION} > 21750
+LDFLAGS+=	-Wl,--build-id=sha1
+.endif
+
 CFLAGS+=	${DEBUG_FLAGS}
 .if ${MACHINE_CPUARCH} == amd64
 CFLAGS+=	-fno-omit-frame-pointer -mno-omit-leaf-frame-pointer