linux/mips: handle wait status 0x7f specially for WIFSIGNALED and WIFSTOPPED

linux/mips: handle wait status 0x7f specially for WIFSIGNALED and WIFSTOPPED

MIPS Linux has SIGRTMAX=127, thus the wait status 0x7f means the program
is terminated by SIGRTMAX, not stopped.

This cannot happen on other ports so make a special version of
waitstatus.h for MIPS to avoid adding redundant calculation to others.
I cannot find a way to use status only once in the expression, so use
inline functions instead of macros to avoid double-evaluating status.

Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Reviewed-by: Florian Weimer <fweimer@redhat.com>