runtime: fix crash trace on amd64

g is not in r15 anymore.
now it's in a per-thread memory segment,
which is valid even inside a signal handler,
so we can just refer to g directly.

Fixes #1082.

R=r
CC=golang-dev
https://golang.org/cl/2149045

src/pkg/runtime/darwin/amd64/signal.c

@@ -112,8 +112,8 @@ sighandler(int32 sig, Siginfo *info, void *context)
 	printf("\n");
 
 	if(gotraceback()){
-		traceback((void*)r->rip, (void*)r->rsp, 0, (void*)r->r15);
-		tracebackothers((void*)r->r15);
+		traceback((void*)r->rip, (void*)r->rsp, 0, g);
+		tracebackothers(g);
 		dumpregs(r);
 	}
 

src/pkg/runtime/freebsd/amd64/signal.c

@@ -106,8 +106,8 @@ sighandler(int32 sig, Siginfo* info, void* context)
 	printf("\n");
 
 	if(gotraceback()){
-		traceback((void*)r->mc_rip, (void*)r->mc_rsp, 0, (void*)r->mc_r15);
-		tracebackothers((void*)r->mc_r15);
+		traceback((void*)r->mc_rip, (void*)r->mc_rsp, 0, g);
+		tracebackothers(g);
 		dumpregs(r);
 	}
 

src/pkg/runtime/linux/amd64/signal.c

@@ -105,8 +105,8 @@ sighandler(int32 sig, Siginfo* info, void* context)
 	printf("\n");
 
 	if(gotraceback()){
-		traceback((void*)r->rip, (void*)r->rsp, 0, (void*)r->r15);
-		tracebackothers((void*)r->r15);
+		traceback((void*)r->rip, (void*)r->rsp, 0, g);
+		tracebackothers(g);
 		dumpregs(r);
 	}