cmd/ld: add .note.GNU-stack section for external linking

Fixes #5392.

R=iant, r
CC=golang-dev
https://golang.org/cl/9119043

src/cmd/ld/elf.c

@@ -930,6 +930,8 @@ doelf(void)
 			addstring(shstrtab, ".rel.noptrdata");
 			addstring(shstrtab, ".rel.data");
 		}
+		// add a .note.GNU-stack section to mark the stack as non-executable
+		addstring(shstrtab, ".note.GNU-stack");
 	}
 
 	if(!debug['s']) {
@@ -1403,8 +1405,13 @@ elfobj:
 			elfshreloc(sect);
 		for(sect=segdata.sect; sect!=nil; sect=sect->next)
 			elfshreloc(sect);
+		// add a .note.GNU-stack section to mark the stack as non-executable
+		sh = elfshname(".note.GNU-stack");
+		sh->type = SHT_PROGBITS;
+		sh->addralign = 1;
+		sh->flags = 0;
 	}
-		
+
 	if(!debug['s']) {
 		sh = elfshname(".symtab");
 		sh->type = SHT_SYMTAB;

src/pkg/runtime/cgo/gcc_386.S

@@ -40,3 +40,6 @@ EXT(__stack_chk_fail_local):
 1:
 	jmp 1b
 
+#ifdef __ELF__
+.section .note.GNU-stack,"",@progbits
+#endif

src/pkg/runtime/cgo/gcc_amd64.S

@@ -42,3 +42,7 @@ EXT(crosscall_amd64):
 	popq %rbp
 	popq %rbx
 	ret
+
+#ifdef __ELF__
+.section .note.GNU-stack,"",@progbits
+#endif

src/pkg/runtime/cgo/gcc_arm.S

@@ -34,3 +34,6 @@ EXT(__stack_chk_fail_local):
 1:
 	b 1b
 
+#ifdef __ELF__
+.section .note.GNU-stack,"",@progbits
+#endif