Update build.py to build on Darwin

Bug: http://b/28196329

Main changes:
- use cmake and ninja from prebuilts/cmake
- remove hardcoded 'linux-x86' in gcc toolchain's path
- do not build runtimes on Darwin

Test: python llvm/android/build.py passes on Linux and Darwin

Change-Id: Icf3e42393918f5292b8f77afb7947a9ddac7659e
1 file changed