Previously these used the same "major architecture" names that the
`#if os(...)` query accepts, but that can be a problem when building
for, say, both armv7 and armv7s, even if the content is "the same".
For a transition period where external build tools are involved, the
compiler will look for "arm.swiftmodule" if it fails to find
"armv7.swiftmodule" or any other 32-bit ARM architecture. No other
Apple platform architectures are affected, and AFAIK no one's using
the architecture-based layout on Linux or any other platforms.
rdar://problem/45174692
