Hide the stub created for MO_ExternalSymbol too.

given

declare void @llvm.memset.p0i8.i32(i8* nocapture, i8, i32, i32, i1)
declare void @foo()
define void @bar() {
  call void @foo()
  call void @llvm.memset.p0i8.i32(i8* null, i8 0, i32 188, i32 1, i1 false)
  ret void
}

We used to produce

L_foo$stub:
        .indirect_symbol        _foo
        .ascii  "\364\364\364\364\364"

_memset$stub:
        .indirect_symbol        _memset
        .ascii  "\364\364\364\364\364"

We not produce a private stub for memset too.

Stubs are not needed with recent linkers, but we still produce them for darwin8.

Thanks to David Fang for confirming that gcc used to do this too.