fix mips mcontext_t structure size

otherwise offs in ucontext_t will be wrong, and break code that
inspects or modifies the signal makes (including cancellation code).

arch/mips/bits/signal.h

@@ -2,7 +2,11 @@
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
 typedef struct {
-	unsigned long __regs[21];
+	unsigned __mc1[2];
+	unsigned long long __mc2[65];
+	unsigned __mc3[5];
+	unsigned long long __mc4[2];
+	unsigned __mc5[6];
 } mcontext_t;
 
 typedef struct __ucontext {