[MC] Teach the MachO object writer about N_FUNC_COLD

N_FUNC_COLD is a new MachO symbol attribute. It's a hint to the linker
to order a symbol towards the end of its section, to improve locality.

Example:

```
void a1() {}
__attribute__((cold)) void a2() {}
void a3() {}
int main() {
  a1();
  a2();
  a3();
  return 0;
}
```

A linker that supports N_FUNC_COLD will order _a2 to the end of the text
section. From `nm -njU` output, we see:

```
_a1
_a3
_main
_a2
```

Differential Revision: https://reviews.llvm.org/D57190

llvm-svn: 352227