runtime: add conversion specifier to printf for char values

R=r, golang-dev
CC=golang-dev
https://golang.org/cl/7327053

src/pkg/runtime/print.c

@@ -84,6 +84,7 @@ vprintf(int8 *s, byte *base)
 		narg = 0;
 		switch(*p) {
 		case 't':
+		case 'c':
 			narg = arg + 1;
 			break;
 		case 'd':	// 32-bit
@@ -126,6 +127,9 @@ vprintf(int8 *s, byte *base)
 		case 'a':
 			runtime·printslice(*(Slice*)v);
 			break;
+		case 'c':
+			runtime·printbyte(*(int8*)v);
+			break;
 		case 'd':
 			runtime·printint(*(int32*)v);
 			break;
@@ -202,6 +206,12 @@ runtime·printbool(bool v)
 	gwrite((byte*)"false", 5);
 }
 
+void
+runtime·printbyte(int8 c)
+{
+	gwrite(&c, 1);
+}
+
 void
 runtime·printfloat(float64 v)
 {

src/pkg/runtime/runtime.h

@@ -817,6 +817,7 @@ void*	runtime·getcallerpc(void*);
  * runtime go-called
  */
 void	runtime·printbool(bool);
+void	runtime·printbyte(int8);
 void	runtime·printfloat(float64);
 void	runtime·printint(int64);
 void	runtime·printiface(Iface);