s390: Add '-std=gnu11' to decompressor and purgatory CFLAGS

stable inclusion
from stable-v6.6.76
commit cefbf9f892ceaebba2f45c5713f4e9608f7bc266
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/IBW08Q

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cefbf9f892ceaebba2f45c5713f4e9608f7bc266



--------------------------------

commit 3b8b80e993766dc96d1a1c01c62f5d15fafc79b9 upstream.

GCC 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, there are certain
places in the s390 code that use their 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=gnu11' 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.

Cc: stable@vger.kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20250122-s390-fix-std-for-gcc-15-v1-1-8b00cadee083@kernel.org


Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit cefbf9f892ceaebba2f45c5713f4e9608f7bc266)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>