kselftest/arm64: Add a GCS test program built with the system libc

There are things like threads which nolibc struggles with which we want
to add coverage for, and the ABI allows us to test most of these even if
libc itself does not understand GCS so add a test application built
using the system libc.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Tested-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-35-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/gcs/.gitignore

@@ -1 +1,2 @@
 basic-gcs
+libc-gcs

tools/testing/selftests/arm64/gcs/Makefile

@@ -6,7 +6,9 @@
 # nolibc.
 #
 
-TEST_GEN_PROGS := basic-gcs
+TEST_GEN_PROGS := basic-gcs libc-gcs
+
+LDLIBS+=-lpthread
 
 include ../../lib.mk
 

tools/testing/selftests/arm64/gcs/gcs-util.h

@@ -16,6 +16,16 @@
 #define __NR_prctl 167
 #endif
 
+#ifndef NT_ARM_GCS
+#define NT_ARM_GCS 0x410
+
+struct user_gcs {
+	__u64 features_enabled;
+	__u64 features_locked;
+	__u64 gcspr_el0;
+};
+#endif
+
 /* Shadow Stack/Guarded Control Stack interface */
 #define PR_GET_SHADOW_STACK_STATUS	74
 #define PR_SET_SHADOW_STACK_STATUS      75