Makefile.inc1: Make sure sub-makes see MK_CLANG_BOOTSTRAP=no when XCC is a path

Currently we override MK_CLANG_BOOTSTRAP to no so we don't build a
bootstrap compiler, but subdirectories don't see that and so the hack in
bsd.sys.mk to prefer our includes over Clang's resource dir for external
toolchains is not enabled unless you use -DWITHOUT_CLANG_BOOTSTRAP
explicitly on top of XCC (which tools/build/make.py does not do),
causing duplicate definition errors when building rtld-elf due to the
use of -ffreestanding (Clang's stdint.h will use the system one when
hosted, but its own when freestanding, and only has glibc's preprocessor
guards, not FreeBSD's).

This broke when dropping CLANG_BOOTSTRAP from BROKEN_OPTIONS.

Fixes:		31ba4ce889 ("Allow bootstrapping llvm-tblgen on macOS and Linux")
MFC after:	1 week
Reviewed by:	imp, arichardson
Differential Revision:	https://reviews.freebsd.org/D31529

Makefile.inc1

@@ -93,6 +93,9 @@ X${COMPILER}?=	${${COMPILER}}
 # a cross compiler.
 .if ${XCC:N${CCACHE_BIN}:M/*}
 MK_CLANG_BOOTSTRAP=	no
+# Make sure sub-makes see the option as disabled so the hack in bsd.sys.mk to
+# work around incompatible headers in Clang's resource directory is enabled.
+.MAKEOVERRIDES+=	MK_CLANG_BOOTSTRAP
 .endif
 
 # Pull in compiler metadata from buildworld/toolchain if possible to avoid