runtime: mark sysAlloc nosplit

sysAlloc is the only mem function called from Go.

LGTM=iant, khr
R=golang-codereviews, khr, 0intro, iant
CC=dvyukov, golang-codereviews, r
https://golang.org/cl/139210043

src/pkg/runtime/mem_darwin.c

@@ -7,7 +7,9 @@
 #include "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 #include "malloc.h"
+#include "../../cmd/ld/textflag.h"
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {

src/pkg/runtime/mem_dragonfly.c

@@ -7,12 +7,14 @@
 #include "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 #include "malloc.h"
+#include "textflag.h"
 
 enum
 {
 	ENOMEM = 12,
 };
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {

src/pkg/runtime/mem_freebsd.c

@@ -7,12 +7,14 @@
 #include "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 #include "malloc.h"
+#include "textflag.h"
 
 enum
 {
 	ENOMEM = 12,
 };
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {

src/pkg/runtime/mem_linux.c

@@ -7,6 +7,7 @@
 #include "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 #include "malloc.h"
+#include "../../cmd/ld/textflag.h"
 
 enum
 {
@@ -57,6 +58,7 @@ mmap_fixed(byte *v, uintptr n, int32 prot, int32 flags, int32 fd, uint32 offset)
 	return p;
 }
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {

src/pkg/runtime/mem_nacl.c

@@ -7,12 +7,14 @@
 #include "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 #include "malloc.h"
+#include "../../cmd/ld/textflag.h"
 
 enum
 {
 	Debug = 0,
 };
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {

src/pkg/runtime/mem_netbsd.c

@@ -7,12 +7,14 @@
 #include "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 #include "malloc.h"
+#include "../../cmd/ld/textflag.h"
 
 enum
 {
 	ENOMEM = 12,
 };
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {

src/pkg/runtime/mem_openbsd.c

@@ -7,12 +7,14 @@
 #include "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 #include "malloc.h"
+#include "../../cmd/ld/textflag.h"
 
 enum
 {
 	ENOMEM = 12,
 };
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {

src/pkg/runtime/mem_plan9.c

@@ -7,6 +7,7 @@
 #include "arch_GOARCH.h"
 #include "malloc.h"
 #include "os_GOOS.h"
+#include "../../cmd/ld/textflag.h"
 
 extern byte runtime·end[];
 static byte *bloc = { runtime·end };
@@ -32,17 +33,40 @@ brk(uintptr nbytes)
 	bloc = (byte*)bl + nbytes;
 	runtime·unlock(&memlock);
 	return (void*)bl;	
-	
 }
 
-void*
-runtime·sysAlloc(uintptr nbytes, uint64 *stat)
+static void
+sysalloc(void)
 {
+	uintptr nbytes;
+	uint64 *stat;
 	void *p;
 
+	nbytes = g->m->scalararg[0];
+	stat = g->m->ptrarg[0];
+	g->m->scalararg[0] = 0;
+	g->m->ptrarg[0] = nil;
+
 	p = brk(nbytes);
 	if(p != nil)
 		runtime·xadd64(stat, nbytes);
+
+	g->m->ptrarg[0] = p;
+}
+
+#pragma textflag NOSPLIT
+void*
+runtime·sysAlloc(uintptr nbytes, uint64 *stat)
+{
+	void (*fn)(void);
+	void *p;
+
+	g->m->scalararg[0] = nbytes;
+	g->m->ptrarg[0] = stat;
+	fn = sysalloc;
+	runtime·onM(&fn);
+	p = g->m->ptrarg[0];
+	g->m->ptrarg[0] = nil;
 	return p;
 }
 

src/pkg/runtime/mem_solaris.c

@@ -7,12 +7,14 @@
 #include "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 #include "malloc.h"
+#include "../../cmd/ld/textflag.h"
 
 enum
 {
 	ENOMEM = 12,
 };
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {

src/pkg/runtime/mem_windows.c

@@ -7,6 +7,7 @@
 #include "os_GOOS.h"
 #include "defs_GOOS_GOARCH.h"
 #include "malloc.h"
+#include "../../cmd/ld/textflag.h"
 
 enum {
 	MEM_COMMIT = 0x1000,
@@ -25,6 +26,7 @@ extern void *runtime·VirtualAlloc;
 extern void *runtime·VirtualFree;
 extern void *runtime·VirtualProtect;
 
+#pragma textflag NOSPLIT
 void*
 runtime·sysAlloc(uintptr n, uint64 *stat)
 {