xcompile: replace -print-librt-file-name with -print-libgcc-file-name

The former only exists with a custom patch while the latter is supported
by clang and in the absense of libgcc even points to clang's own runtime
libraries.

Change-Id: I1e30d5518cf78e1d66925d6f2ccada60a43bb4f8
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/19658
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>

util/xcompile/xcompile

@@ -207,7 +207,7 @@ detect_special_flags() {
 
 detect_compiler_runtime() {
 	test -z "$CLANG" || \
-	CC_RT_CLANG="$(${CLANG} ${CFLAGS_CLANG} -print-librt-file-name 2>/dev/null)"
+	CC_RT_CLANG="$(${CLANG} -print-libgcc-file-name 2>/dev/null)"
 	test -z "$GCC" || \
 	CC_RT_GCC="$(${GCC} ${CFLAGS_GCC} -print-libgcc-file-name)"
 }