Define TARGET_CPUARCH and use in libclang_rt as the basis for CRTARCH

When cross-compiling, the wrong architecture was being embedded in the
libclang_rt binary filenames. It should be based on TARGET_ARCH (target), not
MACHINE_ARCH (host).

This is a draft commit against my project branch. Will fix on ^/head soon.

PR:	222925

lib/libclang_rt/Makefile.inc

@@ -2,7 +2,7 @@
 
 .include <src.opts.mk>
 
-CRTARCH=	${MACHINE_CPUARCH:C/amd64/x86_64/}
+CRTARCH=	${TARGET_CPUARCH:C/amd64/x86_64/}
 CRTSRC=		${SRCTOP}/contrib/compiler-rt
 
 .PATH:		${CRTSRC}/lib

share/mk/sys.mk

@@ -15,6 +15,7 @@ unix		?=	We run FreeBSD, not UNIX.
 #
 __TO_CPUARCH=C/mips(n32|64)?(el)?(hf)?/mips/:C/arm(v[67])?(eb)?/arm/:C/powerpc(64|spe)/powerpc/:C/riscv64(sf)?/riscv/
 MACHINE_CPUARCH=${MACHINE_ARCH:${__TO_CPUARCH}}
+TARGET_CPUARCH=${TARGET_ARCH:${__TO_CPUARCH}}
 .endif