target/arm: Unify checking for M Main Extension in MRS/MSR

target/arm: Unify checking for M Main Extension in MRS/MSR

BASEPRI, FAULTMASK, and their _NS equivalents only exist on devices with
the Main Extension.  However, the MRS instruction did not check this,
and the MSR instruction handled it inconsistently (warning BASEPRI, but
silently ignoring writes to BASEPRI_NS).  Unify this behavior and always
warn when reading or writing any of these registers if the extension is
not present.

Signed-off-by: David Reiss <dreiss@meta.com>
Message-id: 167330628518.10497.13100425787268927786-0@git.sr.ht
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>