runtime: implement string ops in Go

Also implement go:nosplit annotation.  Not really needed
for now, but we'll definitely need it for other conversions.

benchmark                 old ns/op     new ns/op     delta
BenchmarkRuneIterate      534           474           -11.24%
BenchmarkRuneIterate2     535           470           -12.15%

LGTM=bradfitz
R=golang-codereviews, dave, bradfitz, minux
CC=golang-codereviews
https://golang.org/cl/93380044

src/pkg/runtime/asm_amd64.s

diff --git a/src/pkg/runtime/asm_amd64.s b/src/pkg/runtime/asm_amd64.s
index b352a50..d15b372 100644
--- a/src/pkg/runtime/asm_amd64.s
+++ b/src/pkg/runtime/asm_amd64.s
@@ -858,6 +858,12 @@
 	MOVQ	-8(AX),AX		// get calling pc
 	RET
 
+TEXT runtime·gogetcallerpc(SB),NOSPLIT,$0-8
+	MOVQ	x+0(FP),AX		// addr of first arg
+	MOVQ	-8(AX),AX		// get calling pc
+	MOVQ	AX,r+4(FP)
+	RET
+
 TEXT runtime·setcallerpc(SB),NOSPLIT,$0-16
 	MOVQ	x+0(FP),AX		// addr of first arg
 	MOVQ	x+8(FP), BX