[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, 5 insertions, 0 deletions

diff --git a/llvm/tools/llvm-nm/llvm-nm.cpp b/llvm/tools/llvm-nm/llvm-nm.cpp
index 08ea238ac3a..f81e909c924 100644
--- a/llvm/tools/llvm-nm/llvm-nm.cpp
+++ b/llvm/tools/llvm-nm/llvm-nm.cpp
@@ -554,6 +554,11 @@ static void darwinPrintSymbol(SymbolicFile &Obj, SymbolListT::iterator I,
       (NDesc & MachO::N_ALT_ENTRY) == MachO::N_ALT_ENTRY)
     outs() << "[alt entry] ";
 
+  if (Filetype == MachO::MH_OBJECT &&
+      ((NType & MachO::N_TYPE) != MachO::N_UNDF) &&
+      (NDesc & MachO::N_COLD_FUNC) == MachO::N_COLD_FUNC)
+    outs() << "[cold] ";
+
   if ((NDesc & MachO::N_ARM_THUMB_DEF) == MachO::N_ARM_THUMB_DEF)
     outs() << "[Thumb] ";