target/arm: Support EL0 v7m msr/mrs for CONFIG_USER_ONLY

target/arm: Support EL0 v7m msr/mrs for CONFIG_USER_ONLY

Simply moving the non-stub helper_v7m_mrs/msr outside of
!CONFIG_USER_ONLY is not an option, because of all of the
other system-mode helpers that are called.

But we can split out a few subroutines to handle the few
EL0 accessible registers without duplicating code.

Reported-by: Christophe Lyon <christophe.lyon@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20191118194916.3670-1-richard.henderson@linaro.org
[PMM: deleted now-redundant comment; added a default case
 to switch in v7m_msr helper]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>