treewide: Work around GCC 15 Werror=unterminated-string-initialization

GCC 15 added a new `unterminated-string-initialization` warning. Even
though crossgcc is still using GCC 14, some Linux distributions (e.g.
Arch Linux) already started shipping GCC 15. Given that coreboot uses
`-Werror` (warnings are errors), this new warning causes build errors
for things built using the host toolchain, such as utilities. In this
case, cbfstool is affected, which prevents building coreboot images.

The nonstring attribute is used to tell the compiler whether or not a
string is intentionally not null terminated. Since the attribute is
only included in GCC 15 for multidimensional character arrays (and even
later for clang) we need to check the GCC version before using the
attribute.

On GCC version prior to GCC 15 the nonstring attribute will not be used,
but that is not a problem since the unterminated-string-initialization
warning only exists since GCC 15. So you can still build on all GCC
versions as before. This way it also works if your host toolchain is GCC
15 (which builds commonlib code for cbfstool) and your coreboot cross
toolchain is GCC 14 (which builds commonlib code for coreboot).
Clang is a diffent matter. According to the documentation, the nonstring
attribute only exists in version 21 which is not yet released by LLVM.

TEST=Build qemu/Q35 successfully

Change-Id: I919d71cb2811e91869ba1ff493a0719ddcc86c36
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87825
Reviewed-by: Benjamin Doron <benjamin.doron00@gmail.com>
Reviewed-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/commonlib/bsd/include/commonlib/bsd/compiler.h

@@ -66,6 +66,13 @@
 #define __printf(a, b) __attribute__((format(printf, a, b)))
 #endif
 
+// This checks the support for the nonstring attribute on multidimensional character arrays.
+#if (defined(__GNUC__) && __GNUC__ >= 15) || (defined(__clang__) && __clang_major__ >= 21)
+#define __nonstring __attribute__((__nonstring__))
+#else
+#define __nonstring
+#endif
+
 /*
  * This evaluates to the type of the first expression, unless that is constant
  * in which case it evaluates to the type of the second. This is useful when

src/commonlib/include/commonlib/loglevel.h

@@ -165,7 +165,7 @@
  */
 #define BIOS_LOG_PREFIX_PATTERN "[%.5s]  "
 #define BIOS_LOG_PREFIX_MAX_LEVEL BIOS_SPEW
-static const char bios_log_prefix[BIOS_LOG_PREFIX_MAX_LEVEL + 1][5] = {
+static const char __nonstring bios_log_prefix[BIOS_LOG_PREFIX_MAX_LEVEL + 1][5] = {
 	/* Note: These strings are *not* null-terminated to save space. */
 	[BIOS_EMERG]   = "EMERG",
 	[BIOS_ALERT]   = "ALERT",

util/cbfstool/common.c

@@ -192,7 +192,7 @@ uint64_t intfiletype(const char *name)
 
 char *bintohex(uint8_t *data, size_t len)
 {
-	static const char translate[16] = "0123456789abcdef";
+	static const char __nonstring translate[16] = "0123456789abcdef";
 
 	char *result = malloc(len * 2 + 1);
 	if (result == NULL)