runtime: export PCDATA value reader

This interface is required to use the PCDATA interface
implemented in Go 1.2.  While initially entirely private, the
FUNCDATA side of the interface has been made public.  This
change completes the FUNCDATA/PCDATA interface.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13735043

src/pkg/runtime/runtime.h

@@ -802,6 +802,7 @@ int32	runtime·funcline(Func*, uintptr, String*);
 int32	runtime·funcarglen(Func*, uintptr);
 int32	runtime·funcspdelta(Func*, uintptr);
 int8*	runtime·funcname(Func*);
+int32	runtime·pcdatavalue(Func*, int32, uintptr);
 void*	runtime·stackalloc(uint32);
 void	runtime·stackfree(void*, uintptr);
 MCache*	runtime·allocmcache(void);

src/pkg/runtime/symtab.c

@@ -208,8 +208,8 @@ runtime·funcspdelta(Func *f, uintptr targetpc)
 	return x;
 }
 
-static int32
-pcdatavalue(Func *f, int32 table, uintptr targetpc)
+int32
+runtime·pcdatavalue(Func *f, int32 table, uintptr targetpc)
 {
 	if(table < 0 || table >= f->npcdata)
 		return -1;
@@ -221,7 +221,7 @@ runtime·funcarglen(Func *f, uintptr targetpc)
 {
 	if(targetpc == f->entry)
 		return 0;
-	return pcdatavalue(f, PCDATA_ArgSize, targetpc-PCQuantum);
+	return runtime·pcdatavalue(f, PCDATA_ArgSize, targetpc-PCQuantum);
 }
 
 void