Clang: Only build needed target for bootstrap compiler.

This will disable the new LLVM_TARGET_ALL option which will only
enable the required target.

This only impacts the bootstrap compiler in WORLDTMP, not the target compiler
that will be installed.

MFC after:	2 weeks
Reviewed by:	sbruno, dim (earlier version)
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D16021

Makefile.inc1

@@ -686,6 +686,9 @@ TMAKE=		\
 XMAKE=		${BMAKE} \
 		TARGET=${TARGET} TARGET_ARCH=${TARGET_ARCH} \
 		MK_GDB=no MK_TESTS=no
+.if empty(.MAKEOVERRIDES:MMK_LLVM_TARGET_ALL)
+XMAKE+=		MK_LLVM_TARGET_ALL=no
+.endif
 
 # kernel-tools stage
 KTMAKEENV=	INSTALL="sh ${.CURDIR}/tools/install.sh" \

tools/build/options/WITH_LLVM_TARGET_ALL

@@ -1,2 +1,4 @@
 .\" $FreeBSD$
 Set to build support for all LLVM targets.
+This option is always applied to the bootstrap compiler for buildworld when
+LLVM is used.