target/microblaze: Fix width of FSR

target/microblaze: Fix width of FSR

The exception status register is only 32-bits wide.  Do not use a
64-bit type to represent it.  Since cpu_fsr is only used during
MSR and MTR instructions, we can just as easily use an explicit
load and store, so eliminate the variable.

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>