[dexter] Force dexter tests to use the host triple

If the default target and host triple don't match then print the following
message when the lit test runs:

    Forcing dexter tests to use host triple {HOST_TRIPLE}.

If we can't target the host arch then, when lit runs, we mark
the dexter test directories as UNSUPPORTED and print the message:

    Host triple {HOST_TRIPLE} not supported. Skipping dexter tests in
    the debuginfo-tests project.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D96494