cmd/ld: supply -s to gcc if -s is passed.

Fixes #5463.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/9239045

src/cmd/ld/elf.c

@@ -909,6 +909,7 @@ doelf(void)
 	addstring(shstrtab, ".gopclntab");
 	
 	if(linkmode == LinkExternal) {
+		debug_s = debug['s'];
 		debug['s'] = 0;
 		debug['d'] = 1;
 

src/cmd/ld/lib.c

@@ -666,8 +666,11 @@ hostlink(void)
 		argv[argc++] = "-m64";
 		break;
 	}
-	if(!debug['s'])
+	if(!debug['s'] && !debug_s) {
 		argv[argc++] = "-gdwarf-2"; 
+	} else {
+		argv[argc++] = "-s";
+	}
 	if(HEADTYPE == Hdarwin)
 		argv[argc++] = "-Wl,-no_pie,-pagezero_size,4000000";
 	argv[argc++] = "-o";

src/cmd/ld/lib.h

@@ -159,6 +159,7 @@ EXTERN	char*	interpreter;
 EXTERN	char*	tmpdir;
 EXTERN	char*	extld;
 EXTERN	char*	extldflags;
+EXTERN	int	debug_s; // backup old value of debug['s']
 
 enum
 {