KVM: selftests: Define _GNU_SOURCE for all selftests code

Define _GNU_SOURCE is the base CFLAGS instead of relying on selftests to
manually #define _GNU_SOURCE, which is repetitive and error prone.  E.g.
kselftest_harness.h requires _GNU_SOURCE for asprintf(), but if a selftest
includes kvm_test_harness.h after stdio.h, the include guards result in
the effective version of stdio.h consumed by kvm_test_harness.h not
defining asprintf():

  In file included from x86_64/fix_hypercall_test.c:12:
  In file included from include/kvm_test_harness.h:11:
 ../kselftest_harness.h:1169:2: error: call to undeclared function
  'asprintf'; ISO C99 and later do not support implicit function declarations
  [-Wimplicit-function-declaration]
   1169 |         asprintf(&test_name, "%s%s%s.%s", f->name,
        |         ^

When including the rseq selftest's "library" code, #undef _GNU_SOURCE so
that rseq.c controls whether or not it wants to build with _GNU_SOURCE.

Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20240423190308.2883084-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/Makefile

@@ -226,8 +226,8 @@ LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include
 endif
 CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
 	-Wno-gnu-variable-sized-type-not-at-end -MD -MP -DCONFIG_64BIT \
-	-fno-builtin-memcmp -fno-builtin-memcpy -fno-builtin-memset \
-	-fno-builtin-strnlen \
+	-D_GNU_SOURCE -fno-builtin-memcmp -fno-builtin-memcpy \
+	-fno-builtin-memset -fno-builtin-strnlen \
 	-fno-stack-protector -fno-PIE -I$(LINUX_TOOL_INCLUDE) \
 	-I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \
 	-I$(<D) -Iinclude/$(ARCH_DIR) -I ../rseq -I.. $(EXTRA_CFLAGS) \

tools/testing/selftests/kvm/aarch64/arch_timer.c

@@ -5,8 +5,6 @@
  *
  * Copyright (c) 2021, Google LLC.
  */
-#define _GNU_SOURCE
-
 #include "arch_timer.h"
 #include "delay.h"
 #include "gic.h"

tools/testing/selftests/kvm/aarch64/page_fault_test.c

@@ -7,7 +7,6 @@
  * hugetlbfs with a hole). It checks that the expected handling method is
  * called (e.g., uffd faults with the right address and write/read flag).
  */
-#define _GNU_SOURCE
 #include <linux/bitmap.h>
 #include <fcntl.h>
 #include <test_util.h>

tools/testing/selftests/kvm/aarch64/psci_test.c

@@ -10,9 +10,6 @@
  *  - A test for KVM's handling of PSCI SYSTEM_SUSPEND and the associated
  *    KVM_SYSTEM_EVENT_SUSPEND UAPI.
  */
-
-#define _GNU_SOURCE
-
 #include <linux/psci.h>
 
 #include "kvm_util.h"

tools/testing/selftests/kvm/aarch64/vgic_init.c

@@ -4,7 +4,6 @@
  *
  * Copyright (C) 2020, Red Hat, Inc.
  */
-#define _GNU_SOURCE
 #include <linux/kernel.h>
 #include <sys/syscall.h>
 #include <asm/kvm.h>

tools/testing/selftests/kvm/arch_timer.c

@@ -19,9 +19,6 @@
  *
  * Copyright (c) 2021, Google LLC.
  */
-
-#define _GNU_SOURCE
-
 #include <stdlib.h>
 #include <pthread.h>
 #include <linux/sizes.h>

tools/testing/selftests/kvm/demand_paging_test.c

@@ -6,9 +6,6 @@
  * Copyright (C) 2018, Red Hat, Inc.
  * Copyright (C) 2019, Google, Inc.
  */
-
-#define _GNU_SOURCE /* for pipe2 */
-
 #include <inttypes.h>
 #include <stdio.h>
 #include <stdlib.h>

tools/testing/selftests/kvm/dirty_log_test.c

@@ -4,9 +4,6 @@
  *
  * Copyright (C) 2018, Red Hat, Inc.
  */
-
-#define _GNU_SOURCE /* for program_invocation_name */
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <pthread.h>

tools/testing/selftests/kvm/guest_memfd_test.c

@@ -4,8 +4,6 @@
  *
  * Author: Chao Peng <chao.p.peng@linux.intel.com>
  */
-
-#define _GNU_SOURCE
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>

tools/testing/selftests/kvm/hardware_disable_test.c

@@ -4,9 +4,6 @@
  * kvm_arch_hardware_disable is called and it attempts to unregister the user
  * return notifiers.
  */
-
-#define _GNU_SOURCE
-
 #include <fcntl.h>
 #include <pthread.h>
 #include <semaphore.h>