kselftest/arm64: Enable GCS for the FP stress tests

While it's a bit off topic for them the floating point stress tests do give
us some coverage of context thrashing cases, and also of active signal
delivery separate to the relatively complicated framework in the actual
signals tests. Have the tests enable GCS on startup, ignoring failures so
they continue to work as before on systems without GCS.

Reviewed-by: Thiago Jung Bauermann <[e-mail]>
Tested-by: Thiago Jung Bauermann <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Catalin Marinas <[e-mail]>

tools/testing/selftests/arm64/fp/assembler.h

···
 	bl	puts
 .endm
 
+#define PR_SET_SHADOW_STACK_STATUS      75
+# define PR_SHADOW_STACK_ENABLE         (1UL << 0)
+
+.macro enable_gcs
+	// Run with GCS
+	mov	x0, PR_SET_SHADOW_STACK_STATUS
+	mov	x1, PR_SHADOW_STACK_ENABLE
+	mov	x2, xzr
+	mov	x3, xzr
+	mov	x4, xzr
+	mov	x5, xzr
+	mov	x8, #__NR_prctl
+	svc	#0
+.endm
+
 #endif /* ! ASSEMBLER_H */

tools/testing/selftests/arm64/fp/fpsimd-test.S

···
 // Main program entry point
 .globl _start
 function _start
+	enable_gcs
+
 	mov	x23, #0		// signal count
 
 	mov	w0, #SIGINT

tools/testing/selftests/arm64/fp/sve-test.S

···
 // Main program entry point
 .globl _start
 function _start
+	enable_gcs
+
 	mov	x23, #0		// Irritation signal count
 
 	mov	w0, #SIGINT

tools/testing/selftests/arm64/fp/za-test.S

···
 // Main program entry point
 .globl _start
 function _start
+	enable_gcs
+
 	mov	x23, #0		// signal count
 
 	mov	w0, #SIGINT

tools/testing/selftests/arm64/fp/zt-test.S

···
 // Main program entry point
 .globl _start
 function _start
+	enable_gcs
+
 	mov	x23, #0		// signal count
 
 	mov	w0, #SIGINT