target/microblaze: Tidy raising of exceptions

target/microblaze: Tidy raising of exceptions

Split out gen_raise_exception which does no cpu state sync.
Rename t_gen_raise_exception to gen_raise_exception_sync to
emphasize that it does a sync.  Create gen_raise_hw_excp to
simplify code raising EXCP_HW_EXCP.

Since there is now only one use of cpu_esr, perform a store
instead and remove the TCG 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>