gcc-cross-initial: Ensure it uses an isolated sysroot

If we don't do this, a stale limits.h may be detected in STAGING_DIR_TARGET
which would result in a different limits.h getting generated by gcc-cross-initial
that references it. The referenced limits.h will then not get found by eglibc-initial
causing rather strange build failures.

The simplest solution is to create a temporary sysroot containing only the things
gcc-cross-initial should care about and this results in a correct limits.h file
regardless of what else may have been built.

(From OE-Core rev: 9c304eae0724474902fe2f3150adc6af115af9ba)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/gcc/gcc-cross-initial.inc

@@ -19,11 +19,23 @@ EXTRA_OECONF = "--with-newlib \
                 ${OPTSPACE} \
 		--program-prefix=${TARGET_PREFIX} \
 		--with-sysroot=${STAGING_DIR_TARGET} \
-		--with-build-sysroot=${STAGING_DIR_TARGET} \
+		--with-build-sysroot=${GCCCROSS_BUILDSYSROOT} \
 		${EXTRA_OECONF_INITIAL} \
 		${@base_contains('DISTRO_FEATURES', 'ld-is-gold', '--with-ld=${STAGING_BINDIR_TOOLCHAIN}/${TARGET_PREFIX}ld.bfd', '', d)} \
 		${EXTRA_OECONF_FPU}"
 
+
+GCCCROSS_BUILDSYSROOT = "${B}/tmpsysroot"
+
+do_configure_prepend () {
+	sysr=${GCCCROSS_BUILDSYSROOT}${target_includedir}
+	mkdir -p $sysr
+	for t in linux asm asm-generic; do
+		rm -f $sysr/$t
+		ln -s ${STAGING_DIR_TARGET}${target_includedir}/$t $sysr/
+	done
+}
+
 do_compile () {
     oe_runmake all-gcc all-target-libgcc
 }