configs/qemu_mips*: backport a patch for GCC 15.x compatibility

Mips kernel maintainer merged a patch [1] for GCC 15.x compatibility
that will likely be merged upstream soon.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git/commit/?id=0f4ae7c6ecb89bfda026d210dcf8216fb67d2333

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>
(cherry picked from commit 97c7311c64)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

board/qemu/patches/linux/0001-mips-Add-std-flag-specified-in-KBUILD_CFLAGS-to-vdso.patch

@@ -0,0 +1,52 @@
+From c94606e06b54bc7b909fb342aa8dae732b50f5cd Mon Sep 17 00:00:00 2001
+From: Khem Raj <raj.khem@gmail.com>
+Date: Sat, 29 Mar 2025 08:39:03 -0700
+Subject: [PATCH] mips: Add -std= flag specified in KBUILD_CFLAGS to vdso
+ CFLAGS
+
+GCC 15 changed the default C standard dialect from gnu17 to gnu23,
+which should not have impacted the kernel because it explicitly requests
+the gnu11 standard in the main Makefile. However, mips/vdso code uses
+its own CFLAGS without a '-std=' value, which break with this dialect
+change because of the kernel's own definitions of bool, false, and true
+conflicting with the C23 reserved keywords.
+
+  include/linux/stddef.h:11:9: error: cannot use keyword 'false' as enumeration constant
+     11 |         false   = 0,
+        |         ^~~~~
+  include/linux/stddef.h:11:9: note: 'false' is a keyword with '-std=c23' onwards
+  include/linux/types.h:35:33: error: 'bool' cannot be defined via 'typedef'
+     35 | typedef _Bool                   bool;
+        |                                 ^~~~
+  include/linux/types.h:35:33: note: 'bool' is a keyword with '-std=c23' onwards
+
+Add -std as specified in KBUILD_CFLAGS to the decompressor and purgatory
+CFLAGS to eliminate these errors and make the C standard version of these
+areas match the rest of the kernel.
+
+Upstream: https://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git/commit/?id=0f4ae7c6ecb89bfda026d210dcf8216fb67d2333
+
+Signed-off-by: Khem Raj <raj.khem@gmail.com>
+Cc: stable@vger.kernel.org
+Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
+[Romain: backport to 6.12.27]
+Signed-off-by: Romain Naour <romain.naour@smile.fr>
+---
+ arch/mips/vdso/Makefile | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/arch/mips/vdso/Makefile b/arch/mips/vdso/Makefile
+index b289b2c1b294..c729bd687804 100644
+--- a/arch/mips/vdso/Makefile
++++ b/arch/mips/vdso/Makefile
+@@ -27,6 +27,7 @@ endif
+ # offsets.
+ cflags-vdso := $(ccflags-vdso) \
+ 	$(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
++	$(filter -std=%,$(KBUILD_CFLAGS)) \
+ 	-O3 -g -fPIC -fno-strict-aliasing -fno-common -fno-builtin -G 0 \
+ 	-mrelax-pic-calls $(call cc-option, -mexplicit-relocs) \
+ 	-fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \
+-- 
+2.49.0
+