target/arm: remove run-time semihosting checks for linux-user

target/arm: remove run-time semihosting checks for linux-user

Now we do all our checking at translate time we can make cpu_loop a
little bit simpler. We also introduce a simple linux-user semihosting
test case to defend the functionality. The out-of-tree softmmu based
semihosting tests are still more comprehensive.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20190913151845.12582-6-alex.bennee@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>