Use prebuilt gcc for MacOSX

1. Use prebuilts/gcc/darwin-x86/host/i686-apple-darwin-4.2.1
   See https://android-review.googlesource.com/#/c/46223/
2. Rewrite logic dealing with mac_sdk_version to support all
   MacOSX SDK 10.6, 10.7 and 10.8.  Note that since
   ad2342375963c2468849c1f83a97158383db6511 emulator no longer
   depends on 10.6 to build.  Since the lowest SDK among intersection
   of the "available" and the "supported" SDKs is picked, add a
   new variable MAC_SDK_VERSION for developer really want to overwrite
   it.  MAC_SDK_VERSION still has to be one of the supported, though.
3. Improve mac_sdk_path detection to deal with case where Xcode
   *dmg is mounted only, not installed at /Applications.
4. Now we can retire BUILD_MAC_SDK_EXPERIMENTAL

Change-Id: I83e463556a857d527710f766de0e19e1b576151f