mips: Add -std= flag specified in KBUILD_CFLAGS to vdso CFLAGS

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.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>