target/microblaze: Split out MSR[C] to its own variable

target/microblaze: Split out MSR[C] to its own variable

Having the MSR[C] bit separate will improve arithmetic that operates
on the carry bit.  Having mb_cpu_read_msr() populate MSR[CC] will
prevent the carry copy not matching the carry bit.

Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>