This adds all the permutations for tests as make targets of the form
test-libcxx-(host|target)-(clang|gcc)-(32|64).
This also changes the host tests to use the Android build system (like
I had done with https://android-review.googlesource.com/#/c/111924/).
This probably should have been a separate patch, but I got carried
away (and wanted to make sure this new approach would work for both),
and now they're non-trivial to split.
Change-Id: Ie99caf6c3ff21c833408f99d37299d966ee7bc94
78f8c1981e