lib/atoi/a2i.[ch]: Add const-generic macros

lib/atoi/a2i.[ch]: Add const-generic macros

These overloaded macros allow passing either a const or a non-const
endp, and will call the appropriate function.  This kind of const
overloading has prior art in C23's string functions, such as memchr(3).

Martin suggested using an artificial function pointer in _Generic(3); it
allows switching on various types at the same time.

Also add a comment referring to liba2i's PDF manual for documentation.

Link: <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf#subsubsection.7.26.5.2>
Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114731>
Link: <http://www.alejandro-colomar.es/share/dist/liba2i/git/HEAD/liba2i-HEAD.pdf>
Co-developed-by: Martin Uecker <muecker@gwdg.de>
Signed-off-by: Alejandro Colomar <alx@kernel.org>