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

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: a985fe638344 ("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>