Reorganize carry to store flags separately instead of part of XER

Also correct behavior with regards to which bits in XER are treated as zero
based on a hwtest (probably doesn't affect any real games, but might as well
be correct).
Implement XER optimization on ARMv7 JIT core.
Merge pull request #1 from Sonicadvance1/xer-optimization-armv7

Implement XER optimization on ARMv7 JIT core.