maint: use a more standard return from mbrtowc test
With this we can use this check in coreutils to find configure issues:
grep -E '(program exited with status 1$|core dumped)' config.log
* m4/mbrtowc.m4 (gl_MBRTOWC_C_LOCALE): Don't return 1
from the test program as this often indicates an
unhandled case in the test program.
(gl_MBRTOWC_INCOMPLETE_STATE): Likewise.
(gl_MBRTOWC_SANITYCHECK): Likewise.
(gl_MBRTOWC_NULL_ARG2): Likewise.
(gl_MBRTOWC_NUL_RETVAL): Likewise.