META_MODE: Pass along the default sysroot in bootstrap-tools to avoid rebuilds later.

Some of the clang libraries build in this phase and the cross-tools
phase.  Later in the cross-tools phase when they build with a default
TOOLS_PREFIX, they see a changed build command in meta mode due to
the changed DEFAULT_SYSROOT.  This is avoided by passing along
TOOLS_PREFIX earlier.

Sponsored by:	EMC / Isilon Storage Division

Makefile.inc1

@@ -311,6 +311,7 @@ CROSSENV+=	${TARGET_CFLAGS}
 
 # bootstrap-tools stage
 BMAKEENV=	INSTALL="sh ${.CURDIR}/tools/install.sh" \
+		TOOLS_PREFIX=${WORLDTMP} \
 		PATH=${BPATH}:${PATH} \
 		WORLDTMP=${WORLDTMP} \
 		MAKEFLAGS="-m ${.CURDIR}/tools/build/mk ${.MAKEFLAGS}"