[opt] Report if the provided architecture is invalid.

Partly addresses PR15026.

There are a few tests that passed in invalid architectures, which are fixed in: rL355349 and D58931

Reviewers: echristo, efriedma, rengolin, atrick

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D58933

llvm-svn: 355455