MINOR: compiler: add __nonstring macro

MINOR: compiler: add __nonstring macro

GCC 15 throws the following warning on fixed-size char arrays if they do not
contain terminated NUL:

src/tools.c:2041:25: error: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (17 chars into 16 available) [-Werror=unterminated-string-initialization]
 2041 | const char hextab[16] = "0123456789ABCDEF";

We are using a couple of such definitions for some constants. Converting them
to flexible arrays, like: hextab[] = "0123456789ABCDEF" may have consequences,
as enlarged arrays won't fit anymore where they were possibly located due to
the memory alignement constraints.

GCC adds 'nonstring' variable attribute for such char arrays, but clang and
other compilers don't have it. Let's wrap 'nonstring' with our
__nonstring macro, which will test if the compiler supports this attribute.

This fixes the issue #2910.