[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

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index aed46d58f99..4c9fe59af42 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -663,6 +663,9 @@ void AsmPrinter::EmitFunctionHeader() {
   if (MAI->hasDotTypeDotSizeDirective())
     OutStreamer->EmitSymbolAttribute(CurrentFnSym, MCSA_ELF_TypeFunction);
 
+  if (F.hasFnAttribute(Attribute::Cold))
+    OutStreamer->EmitSymbolAttribute(CurrentFnSym, MCSA_Cold);
+
   if (isVerbose()) {
     F.printAsOperand(OutStreamer->GetCommentOS(),
                    /*PrintType=*/false, F.getParent());