xcompile: Fix compiler invocation in testcc

While for GCC targets the compiler is just defined as a single
binary, for clang it is defined as a binary and some options, e.g.:
  clang -target i386-elf -ccc-gcc-name i386-elf-gcc

When executing the compiler with "$1", the shell will look for a
binary with the above name (instead of just clang) and always fail
detection of any CFLAGS.

By adding -c we prevent the compiler from failing because it can't
link a user space program (when what we're looking for, is whether
a specific compiler flag can be used to compile a coreboot object
file)

Change-Id: I1e9ff32fe40efbe3224c69785f31bc277f21d21b
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10816
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/xcompile/xcompile

@@ -72,7 +72,7 @@ testcc() {
 	local tmp_o="$TMPFILE.o"
 	rm -f "$tmp_c" "$tmp_o"
 	echo "void _start(void) {}" >"$tmp_c"
-	"$1" -nostdlib -Werror $2 "$tmp_c" -o "$tmp_o" >/dev/null 2>&1
+	$1 -nostdlib -Werror $2 -c "$tmp_c" -o "$tmp_o" >/dev/null 2>&1
 }
 
 testas() {