MIRI reports data race

[eira-fransham]

Running env MIRIFLAGS=-Zmiri-disable-isolation cargo miri test -- rwlock::tests::test_ruw_arc finds a bug in the test: the line *lock = -1 is reported as a data race. I do not know enough about MIRI or concurrent programming to know what the issue is, but even though the bug is reported in a test it appears to be entirely in safe code which uses the normal parking_lot API. I think this is worth investigating - with my limited knowledge my best guess would be that the LLVM ordering mode is annotated incorrectly somehow.