misc/cgo/testcshared, misc/cgo/testshared: fix clang warnings and errors

Fix several warnings generated on the linux-amd64-clang builder
and make it clear to clang that -znow is a linker only flag.

Tested with

    env CC=clang-3.5 ./all.bash
    env CC=gcc-4.8 ./all.bash

Change-Id: I5ca7366ba8bf6221a36d25a2157dda4b4f3e16fa
Reviewed-on: https://go-review.googlesource.com/9523
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testcshared/main0.c

@@ -5,6 +5,10 @@
 #include <stdint.h>
 #include <stdio.h>
 
+extern int8_t DidInitRun(void);
+extern int8_t DidMainRun(void);
+extern int32_t FromPkg(void);
+
 // Tests libgo.so to export the following functions.
 //   int8_t DidInitRun();
 //   int8_t DidMainRun();

src/cmd/internal/ld/lib.go

@@ -946,7 +946,7 @@ func hostlink() {
 		// We force all symbol resolution to be done at program startup
 		// because lazy PLT resolution can use large amounts of stack at
 		// times we cannot allow it to do so.
-		argv = append(argv, "-znow")
+		argv = append(argv, "-Wl,-znow")
 	}
 
 	argv = append(argv, "-o")