runtime: handle gdb breakpoint in x86 traceback

This lets stack splits work correctly when running under gdb
when gdb has inserted a breakpoint somewhere on the call
stack.

Fixes #6834.

R=golang-codereviews, minux.ma
CC=golang-codereviews
https://golang.org/cl/48650043

src/pkg/runtime/sys_x86.c

@@ -27,7 +27,8 @@ void
 runtime·rewindmorestack(Gobuf *gobuf)
 {
 	byte *pc;
-	
+	Func *f;
+
 	pc = (byte*)gobuf->pc;
 	if(pc[0] == 0xe9) { // jmp 4-byte offset
 		gobuf->pc = gobuf->pc + 5 + *(int32*)(pc+1);
@@ -37,6 +38,13 @@ runtime·rewindmorestack(Gobuf *gobuf)
 		gobuf->pc = gobuf->pc + 2 + *(int8*)(pc+1);
 		return;
 	}
+	if(pc[0] == 0xcc) { // breakpoint inserted by gdb
+		f = runtime·findfunc(gobuf->pc);
+		if(f != nil) {
+			gobuf->pc = f->entry;
+			return;
+		}
+	}
 	runtime·printf("runtime: pc=%p %x %x %x %x %x\n", pc, pc[0], pc[1], pc[2], pc[3], pc[4]);
 	runtime·throw("runtime: misuse of rewindmorestack");
 }