[PATCH] Wrap .subsections_via_symbols directives in #if __APPLE__

[Jon Roelofs]

Reed, care to bikeshed on a name?  I'm drawing a blank.

Another option I considered was to add FILE_LEVEL_DIRECTIVE to the definition of DEFINE_COMPILERRT_PRIVATE_FUNCTION (just like DEFINE_COMPILERRT_FUNCTION has), and delete all of these.  I'm not sure if that is correct though as there are a few other builtins that don't have .subsections_via_symbols.

http://reviews.llvm.org/D3708