runtime: fix struct Sigaction for Linux/ARM

        if we were to use sizeof(sa.sa_mask) instead of 8 as the last argument
        to rt_sigaction, we would have already fixed this bug, so also updated
        Linux/386 and Linux/amd64 files to use that; also test the return value
        of rt_sigaction.

R=dave, rsc
CC=golang-dev
https://golang.org/cl/6297087

src/pkg/runtime/defs_linux_arm.h

@@ -143,6 +143,6 @@ struct Sigaction {
 	void *sa_handler;
 	uint32 sa_flags;
 	void *sa_restorer;
-	uint32 sa_mask;
+	uint64 sa_mask;
 };
 #pragma pack off

src/pkg/runtime/os_linux.h

@@ -10,7 +10,7 @@ int32	runtime·futex(uint32*, int32, uint32, Timespec*, uint32*, uint32);
 int32	runtime·clone(int32, void*, M*, G*, void(*)(void));
 
 struct Sigaction;
-void	runtime·rt_sigaction(uintptr, struct Sigaction*, void*, uintptr);
+int32	runtime·rt_sigaction(uintptr, struct Sigaction*, void*, uintptr);
 void	runtime·setsig(int32, void(*)(int32, Siginfo*, void*, G*), bool);
 void	runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp);
 

src/pkg/runtime/signal_linux_386.c

@@ -129,7 +129,8 @@ runtime·setsig(int32 i, void (*fn)(int32, Siginfo*, void*, G*), bool restart)
 	if(fn == runtime·sighandler)
 		fn = (void*)runtime·sigtramp;
 	sa.k_sa_handler = fn;
-	runtime·rt_sigaction(i, &sa, nil, 8);
+	if(runtime·rt_sigaction(i, &sa, nil, sizeof(sa.sa_mask)) != 0)
+		runtime·throw("rt_sigaction failure");
 }
 
 #define AT_NULL		0

src/pkg/runtime/signal_linux_amd64.c

@@ -139,5 +139,6 @@ runtime·setsig(int32 i, void (*fn)(int32, Siginfo*, void*, G*), bool restart)
 	if(fn == runtime·sighandler)
 		fn = (void*)runtime·sigtramp;
 	sa.sa_handler = fn;
-	runtime·rt_sigaction(i, &sa, nil, 8);
+	if(runtime·rt_sigaction(i, &sa, nil, sizeof(sa.sa_mask)) != 0)
+		runtime·throw("rt_sigaction failure");
 }

src/pkg/runtime/signal_linux_arm.c

@@ -139,7 +139,8 @@ runtime·setsig(int32 i, void (*fn)(int32, Siginfo*, void*, G*), bool restart)
 	if(fn == runtime·sighandler)
 		fn = (void*)runtime·sigtramp;
 	sa.sa_handler = fn;
-	runtime·rt_sigaction(i, &sa, nil, 8);
+	if(runtime·rt_sigaction(i, &sa, nil, sizeof(sa.sa_mask)) != 0)
+		runtime·throw("rt_sigaction failure");
 }
 
 #define AT_NULL		0