semihosting/arm-compat-semi: unify GET/SET_ARG helpers

semihosting/arm-compat-semi: unify GET/SET_ARG helpers

>>>From the semihosting point of view what we want to know is the current
mode of the processor. Unify this into a single helper and allow us to
use the same GET/SET_ARG helpers for the rest of the code. Having the
helper will also be useful later.

Note: we aren't currently testing riscv32 due to missing toolchain for
check-tcg tests.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Message-Id: <20210323165308.15244-7-alex.bennee@linaro.org>