gcc: gcc 6.3.0 fix comparison between pointer and integer

gcc: gcc 6.3.0 fix comparison between pointer and integer

Fix FS#832

/source/build_dir/toolchain-mips_74kc_gcc-6.3.0_musl/gcc-6.3.0/gcc/ubsan.c:
In function 'bool ubsan_use_new_style_p(location_t)':
/source/build_dir/toolchain-mips_74kc_gcc-6.3.0_musl/gcc-6.3.0/gcc/ubsan.c:1474:23:
error: ISO C++ forbids comparison between pointer and integer
[-fpermissive]
       || xloc.file == '\0' || xloc.file[0] == '\xff'
                       ^~~~
make[5]: *** [Makefile:1085: ubsan.o] Error 1

https://www.viva64.com/en/b/0425/#ID0EMGCI

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>
(cherry picked from commit 22e2b402aee17684781ae705a91fb3645299de9c)