locale: Fix --enable-ubsan build failure on some ABIs

locale: Fix --enable-ubsan build failure on some ABIs

On mips, arc, powerpc, and s390 gcc 14 triggers the warning

In function ‘charmap_new_char’,
    inlined from ‘parse_charmap.isra’ at ../locale/programs/charmap.c:570:6:
../locale/programs/charmap.c:1017:32: error: ‘strncmp’ specified bound [2147483649, 4294967295] exceeds maximum object size 2147483647 [-Werror=stringop-overread]
 1017 |   if (cp == &from[len1 - 1] || strncmp (from, to, prefix_len) != 0)
      |                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../locale/programs/charmap.c:1017:32: error: ‘strncmp’ specified bound [2147483649, 4294967295] exceeds maximum object size 2147483647 [-Werror=stringop-overread]
cc1: all warnings being treated as errors

So move the case to an special function and disable the sanitizer.