target/arm: Add PSTATE.BTYPE

target/arm: Add PSTATE.BTYPE

Place this in its own field within ENV, as that will
make it easier to reset from within TCG generated code.

With the change to pstate_read/write, exception entry
and return are automatically handled.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190128223118.5255-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>