Add support for OSX up to 10.10 and CLI Tools

- Support using OSX versions 10.9 and 10.10 as the compilation host.
  Please note that the prebuilt darwin-x86 gcc is missing an upstream
  patch that properly identifies 10.10 as newer than 10.2, which
  restricts mac_sdk_versions_supported to 10.9.
- Support using Xcode Command Line Tools, waiving the requirement for
  installing the full Xcode development suite for 600k worth of C++
  headers.

These build modifications have been verified by successfully compiling a
cm12-hammerhead image from scratch on a OSX Yosemite 10.10.1 host with
Xcode Command Line Tools 6.1.1 installed.

Change-Id: Id1741ef583d186eb6a42093e5111431ae7b08a2c