Force MK_CLANG_IS_CC on in XMAKE.

This ensures that a bootstrap clang compiler is always installed as cc
in WORLDTMP.  If it is only installed as 'clang' then /usr/bin/cc is
used during the build instead of the bootstrap compiler.

Reviewed by:	imp
MFC after:	1 month
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D22332

Makefile.inc1

@@ -734,6 +734,7 @@ TMAKE=		\
 # TOOLS_PREFIX set in BMAKE
 XMAKE=		${BMAKE} \
 		TARGET=${TARGET} TARGET_ARCH=${TARGET_ARCH} \
+		MK_CLANG_IS_CC=yes \
 		MK_GDB=no MK_TESTS=no
 
 # kernel-tools stage