buildgcc: Add coreboot toolchain string to clang version

clang version now returns:
coreboot toolchain v1.33 November 25th, 2015 clang version 3.6.1
(tags/RELEASE_361/final) (based on LLVM 3.6.1)

Change-Id: I948d7f4d06c244987342cfc7d5c7e728cbed93bd
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12777
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/crossgcc/buildgcc

@@ -501,6 +501,7 @@ build_LLVM() {
 	cd -
 
 	$CMAKE -G "Unix Makefiles" -DCMAKE_INSTALL_PREFIX=$DESTDIR$TARGETDIR \
+		-DCLANG_VENDOR="coreboot toolchain v$CROSSGCC_VERSION $CROSSGCC_DATE - " \
 		-DCMAKE_BUILD_TYPE=Release ../$LLVM_DIR || touch .failed
 	$MAKE $JOBS || touch .failed
 	$MAKE install || touch .failed