i.e.make -f Makefile.standalone check-lit CHECK_LIT_TESTS=tests_lit/llvm2ice_tests/arith.lll

The default will be for the directory to be in subzero but
it's also possible to create an absolute pathname.

Extended this to work with cross tests.

Added some primitive help for the makefile.

make -f Makefile.standalone help
make -f Makefile.standalone help-check-lit
make -f makefile.standalone help-check-xtest

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1582243005 .

Patch from Reed Kotler <rkotlerimgtec@gmail.com>.