Use a got to access a hidden weak undefined on MachO.

Trying to link

__attribute__((weak, visibility("hidden"))) extern int foo;
int *main(void) {
  return &foo;
}

on OS X fails with

ld: 32-bit RIP relative reference out of range (-4294971318 max is +/-2GB): from _main (0x100000FAB) to _foo@0x00001000 (0x00000000) in '_main' from test.o for architecture x86_64

The problem being that 0 cannot be computed as a fixed difference from
%rip. Exactly the same issue exists on ELF and we can use the same
solution.

llvm-svn: 322739