[lit] Stop supporting triple substrings in UNSUPPORTED and XFAIL

AFAICT all in-tree lit tests have been converted to use `target=...`
and so there is no longer any need for triples being special.
Some project config files still define their own features based on
the triple, but those are normal feature words (although now are
redundant with target= checks).

Downstream tests that use triple substrings will need to convert.
For example:
    UNSUPPORTED: -aix
    XFAIL: arm
becomes
    UNSUPPORTED: target={{.*}}-aix{{.*}}
    XFAIL: target=arm{{.*}}

You can do
    git log --grep "special handling for triples"
to find many examples of updates to the upstream tests.

https://discourse.llvm.org/t/rfc-lits-requires-and-triples/66041

Differential Revision: https://reviews.llvm.org/D141007