cmd/ld: handle weak symbols

compiler_rt introduces a weak and hidden symbol compilerrt_abort_impl
into our pre-linked _all.o object, we have to handle it.

Fixes #4273.

R=iant, rsc, r
CC=golang-dev
https://golang.org/cl/6783050

misc/cgo/test/issue4273.c

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#ifdef __ELF__
+__attribute__((weak))
+__attribute__((visibility("hidden")))
+void _compilerrt_abort_impl(const char *file, int line, const char *func) {
+}
+#endif

misc/cgo/test/issue4273b.c

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#ifdef __ELF__
+extern void _compilerrt_abort_impl(const char *file, int line, const char *func);
+
+void __my_abort(const char *file, int line, const char *func) {
+	_compilerrt_abort_impl(file, line, func);
+}
+#endif

src/cmd/ld/ldelf.c

@@ -807,6 +807,13 @@ readsym(ElfObj *obj, int i, ElfSym *sym, int needSym)
 					s->type = SHIDDEN;
 				}
 			break;
+		case ElfSymBindWeak:
+			if(needSym) {
+				s = newsym(sym->name, 0);
+				if(sym->other == 2)
+					s->type = SHIDDEN;
+			}
+			break;
 		default:
 			werrstr("%s: invalid symbol binding %d", sym->name, sym->bind);
 			return -1;