fix incorrect SIGSTKFLT on all mips archs

signal 7 is SIGEMT on Linux mips* ABI according to the man pages and
kernel. it's not clear where the wrong name came from but it dates
back to original mips commit.

arch/mips/bits/signal.h

@@ -93,7 +93,7 @@ typedef struct __ucontext {
 #define SIGTRAP   5
 #define SIGABRT   6
 #define SIGIOT    SIGABRT
-#define SIGSTKFLT 7
+#define SIGEMT    7
 #define SIGFPE    8
 #define SIGKILL   9
 #define SIGBUS    10

arch/mips64/bits/signal.h

@@ -112,7 +112,7 @@ typedef struct __ucontext {
 #define SIGTRAP   5
 #define SIGABRT   6
 #define SIGIOT    SIGABRT
-#define SIGSTKFLT 7
+#define SIGEMT    7
 #define SIGFPE    8
 #define SIGKILL   9
 #define SIGBUS    10

arch/mipsn32/bits/signal.h

@@ -112,7 +112,7 @@ typedef struct __ucontext {
 #define SIGTRAP   5
 #define SIGABRT   6
 #define SIGIOT    SIGABRT
-#define SIGSTKFLT 7
+#define SIGEMT    7
 #define SIGFPE    8
 #define SIGKILL   9
 #define SIGBUS    10