Derek's CL to check out subzero calls the source directory
"subzero", and the file header comments call the directory
"subzero". Just make the python sys.path munging for
importing pydir more generic.

Also change crosstest to not run the raw LLVM "opt" with
optimizations (only use it for ABI stabilization passes).
Instead run pnacl-clang with -O2. Otherwise, newer NACL_SDK
versions include a newer LLVM "opt" binary which
autovectorizes and may generate vector IR that is not
handled by Subzero yet.

E.g.,
LLVM ERROR: Invalid PNaCl instruction:   %1 = insertelement <4 x i32> undef, i32 %0, i32 0
w/ pepper_canary to version 37, revision 274873

BUG=none
TEST=make -f Makefile.standalone check
R=stichnot@chromium.org, wala@chromium.org

Review URL: https://codereview.chromium.org/317963002