[libc++abi] Replace LIBCXXABI_HAS_NO_EXCEPTIONS by TEST_HAS_NO_EXCEPTIONS

This clarifies the difference between test for exception support in
libc++abi tests and support for exceptions built into libc++abi.
This also removes the rather confusing similarity between the
_LIBCXXABI_NO_EXCEPTIONS and LIBCXXABI_HAS_NO_EXCEPTIONS macros.

Finally, TEST_HAS_NO_EXCEPTIONS is also detected automatically based
on -fno-exceptions, so it doesn't have to be specified explicitly
through Lit's compile_flags.