versioner: cache -cc1 flags generated by the clang Driver.

Profiling showed that majority of time spent by versioner was being
spent in the x86 and x86_64 driver statting random files in /usr/lib,
looking for a toolchain. Hardcode a list of per-target flags which
correspond to a subset of the ones generated by clang, and use those
instead of calling out to Driver.

This changes the result of `time versioner` from:
    versioner  156.57s user 1180.08s system 4109% cpu 32.525 total
to:
    versioner  109.12s user 17.43s system 2433% cpu 5.201 total

Bug: http://b/32748936
Test: python run_tests.py
Change-Id: I7d254a105bf1a283cbba0546321b04e308e703d1