Lintrunner: Enable mypy testing on backends/arm (#7776)

Migration from pyre to mypy in the lintrunner by enabling mypy for
backends/arm.

But, choosing to ignore the directory backends/arm/test.

Adding ignores all over the place. These needs to be fixed properly in
the future, but now we will start to catch new things trying to sneak
in.

Change-Id: Ie7f73d5688aaec3b32dca9f0cd042da94c06f487