kselftest/arm64/gcs/basic-gcs: Respect parent directory CFLAGS

basic-gcs has it's own make rule to handle the special compiler
invocation to build against nolibc. This rule does not respect the
$(CFLAGS) passed by the Makefile from the parent directory.
However these $(CFLAGS) set up the include path to include the UAPI
headers from the current kernel.
Due to this the asm/hwcap.h header is used from the toolchain instead of
the UAPI and the definition of HWCAP_GCS is not found.

Restructure the rule for basic-gcs to respect the $(CFLAGS).
Also drop those options which are already provided by $(CFLAGS).

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Closes: https://lore.kernel.org/lkml/CA+G9fYv77X+kKz2YT6xw7=9UrrotTbQ6fgNac7oohOg8BgGvtw@mail.gmail.com/
Fixes: a985fe6 ("kselftest/arm64/gcs: Use nolibc's getauxval()")
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

Author: t-8ch

tools/testing/selftests/arm64/gcs/Makefile

@@ -14,11 +14,11 @@ LDLIBS+=-lpthread
 include ../../lib.mk
 
 $(OUTPUT)/basic-gcs: basic-gcs.c
-	$(CC) -g -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
-		-static -include ../../../../include/nolibc/nolibc.h \
+	$(CC) $(CFLAGS) -fno-asynchronous-unwind-tables -fno-ident -s -nostdlib -nostdinc \
+		-static -I../../../../include/nolibc -include ../../../../include/nolibc/nolibc.h \
 		-I../../../../../usr/include \
 		-std=gnu99 -I../.. -g \
-		-ffreestanding -Wall $^ -o $@ -lgcc
+		-ffreestanding $^ -o $@ -lgcc
 
 $(OUTPUT)/gcs-stress-thread: gcs-stress-thread.S
 	$(CC) -nostdlib $^ -o $@