[llvm-cov] Assume repeat architectures for universal binaries

In the case you pass multiple universal binaries to llvm-cov, assume
that if only 1 architecture is passed, it should be used for all the
passed binaries.

This makes it easier to use multiple multi-arch binaries, since it's
likely very rare that your architectures mismatch significantly if you
also have multiple binaries in a single llvm-cov invocation. If the
architecture is invalid for any of the passed binaries, it will still
fail later.

Differential Revision: https://reviews.llvm.org/D121667