glibc on mips is missing SIGSTKFLT

Do not therefore assume it being available linuxwide

Fixes
| lib/portability.c:433:3: error: use of undeclared identifier 'SIGSTKFLT'
|   SIGNIFY(STKFLT), SIGNIFY(POLL), SIGNIFY(PWR),
|   ^

Upstream-Status: Submitted [https://github.com/landley/toybox/pull/195]
Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/lib/portability.c
+++ b/lib/portability.c
@@ -430,7 +430,10 @@ static const struct signame signames[] =
   // Non-POSIX signals that cause termination
   SIGNIFY(PROF), SIGNIFY(IO),
 #ifdef __linux__
-  SIGNIFY(STKFLT), SIGNIFY(POLL), SIGNIFY(PWR),
+# if !defined(__GLIBC__) && !defined(__mips__)
+   SIGNIFY(STKFLT),
+# endif
+  SIGNIFY(POLL), SIGNIFY(PWR),
 #elif defined(__APPLE__)
   SIGNIFY(EMT), SIGNIFY(INFO),
 #endif