runtime: mask SSE exceptions on plan9/amd64

The Go run-time assumes that all SSE floating-point exceptions
are masked so that Go programs are not broken by such invalid
operations. By default, the 64-bit version of the Plan 9 kernel
masks only some SSE floating-point exceptions. Here, we mask
them all on a per-thread basis.

R=rsc, rminnich, minux.ma
CC=golang-dev
https://golang.org/cl/6592056

src/pkg/runtime/os_plan9.h

@@ -18,6 +18,7 @@ int32 	runtime·plan9_semrelease(uint32 *addr, int32 count);
 int32	runtime·notify(void (*fn)(void*, byte*));
 int32	runtime·noted(int32);
 void	runtime·gonote(void*, byte*);
+void	runtime·setfpmasks(void);
 
 /* open */
 enum

src/pkg/runtime/sys_plan9_386.s

@@ -110,3 +110,7 @@ TEXT runtime·rfork(SB),7,$0
 	CALL	SI	// fn()
 	CALL	runtime·exit(SB)
 	RET
+
+// Only used by the 64-bit runtime.
+TEXT runtime·setfpmasks(SB),7,$0
+	RET

src/pkg/runtime/sys_plan9_amd64.s

@@ -121,3 +121,12 @@ TEXT runtime·rfork(SB),7,$0
 // This is needed by asm_amd64.s
 TEXT runtime·settls(SB),7,$0
 	RET
+
+TEXT runtime·setfpmasks(SB),7,$8
+	STMXCSR	0(SP)
+	MOVL	0(SP), AX
+	ANDL	$~0x3F, AX
+	ORL	$(0x3F<<7), AX
+	MOVL	AX, 0(SP)
+	LDMXCSR	0(SP)
+	RET

src/pkg/runtime/thread_plan9.c

@@ -14,6 +14,9 @@ int32 runtime·postnote(int32, int8*);
 void
 runtime·minit(void)
 {
+	// Mask all SSE floating-point exceptions
+	// when running on the 64-bit kernel.
+	runtime·setfpmasks();
 }
 
 static int32