[PATCH, committed] PPC405 atomic support (PR target/21760)

[David Edelsohn]

	My main reason for working on PPC405CR is because the processor
erratum previously was supported and the recent changes created a
regression.  If others will not fix the problem, it falls to me and I am
trying to work around the problem and move on.

	PowerPC atomic support can be improved on a longer timeframe.  I
have other projects I want to complete.

	PPC405CR options I have dismissed include:

1) multilib.
2) first class processor using target_flags.
3) separate option like rs6000_isel.

	PPC405CR erratum support is not worth the level of attention this
discussion is generating.

David