Don't force libraries upon static executables.

The linker uses libc++_static but needs to avoid the libc.a
dependency. It does this by setting `stl: "none"` and manually
linking libc++. This behavior matches make.

A better approach would probably be to generalize system_shared_libs
to system_libs and apply those to static executables the same way we
do for dynamic ones, but that's a patch for another day.

Test: make checkbuild
Bug: http://b/34740564
Change-Id: Ie9da0d49a453a220593e8ec2ee721e9af9378007

cc/binary.go

@@ -131,7 +131,7 @@ func (binary *binaryDecorator) linkerDeps(ctx DepsContext, deps Deps) Deps {
 		}
 
 		if binary.static() {
-			if inList("libc++_static", deps.StaticLibs) {
+			if ctx.selectedStl() == "libc++_static" {
 				deps.StaticLibs = append(deps.StaticLibs, "libm", "libc", "libdl")
 			}
 			// static libraries libcompiler_rt, libc and libc_nomalloc need to be linked with