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

Not completely optimized; there's room for improvement here.
Merge pull request #1070 from FioraAeterna/optimizeca4

JIT: Reorganize carry to store flags separately instead of part of XER