linux-user/elfload: Fix GCC 9 build warnings

linux-user/elfload: Fix GCC 9 build warnings

Fix this warning when building with GCC9 on Fedora 30:
In function ‘strncpy’,
    inlined from ‘fill_psinfo’ at /home/alistair/qemu/linux-user/elfload.c:3208:12,
    inlined from ‘fill_note_info’ at /home/alistair/qemu/linux-user/elfload.c:3390:5,
    inlined from ‘elf_core_dump’ at /home/alistair/qemu/linux-user/elfload.c:3539:9:
/usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 16 equals destination size [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <c4d2b1de9efadcf1c900b91361af9302823a72a9.1556666645.git.alistair.francis@wdc.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>