compiler_types: Introduce __nonstring_array

compiler_types: Introduce __nonstring_array

GCC has expanded support of the "nonstring" attribute so that it can be
applied to arrays of character arrays[1], which is needed to identify
correct static initialization of those kinds of objects. Since this was
not supported prior to GCC 15, we need to distinguish the usage of Linux's
existing __nonstring macro for the attribute for non-multi-dimensional
char arrays. Until GCC 15 is the minimum version, use __nonstring_array to
mark arrays of non-string character arrays. (Regular non-string character
arrays can continue to use __nonstring.) Once GCC 15 is the minimum
compiler version we can replace all uses of __nonstring_array with just
__nonstring and remove this macro.

This allows for changes like this:

-static const char table_sigs[][ACPI_NAMESEG_SIZE] __initconst = {
+static const char table_sigs[][ACPI_NAMESEG_SIZE] __nonstring_array __initconst = {
        ACPI_SIG_BERT, ACPI_SIG_BGRT, ACPI_SIG_CPEP, ACPI_SIG_ECDT,

Which will silence the coming -Wunterminated-string-initialization
warnings in GCC 15:

In file included from ../include/acpi/actbl.h:371,                                                                   from ../include/acpi/acpi.h:26,                                                                     from ../include/linux/acpi.h:26,
                 from ../drivers/acpi/tables.c:19:
../include/acpi/actbl1.h:30:33: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (5 chars into 4 available) [-Wunterminated-string-initialization]
   30 | #define ACPI_SIG_BERT           "BERT"  /* Boot Error Record Table */
      |                                 ^~~~~~                                                      ../drivers/acpi/tables.c:400:9: note: in expansion of macro 'ACPI_SIG_BERT'                           400 |         ACPI_SIG_BERT, ACPI_SIG_BGRT, ACPI_SIG_CPEP, ACPI_SIG_ECDT,
      |         ^~~~~~~~~~~~~                                                                       ../include/acpi/actbl1.h:31:33: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (5 chars into 4 available) [-Wunterminated-string-initialization]
   31 | #define ACPI_SIG_BGRT           "BGRT"  /* Boot Graphics Resource Table */
      |                                 ^~~~~~

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117178
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250310214244.work.194-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>