[lit] Add `target=<triple>` as a feature keyword

As proposed first in D107162 and later in discourse at
https://discourse.llvm.org/t/rfc-lits-requires-and-triples/66041

Modified a couple of lit's own tests to use this; left others as-is,
because for now triple substrings still work in UNSUPPORTED/XFAIL.

Differential Revision: https://reviews.llvm.org/D137434