target/arm: Export arm_v7m_mrs_control

target/arm: Export arm_v7m_mrs_control

Allow the function to be used outside of m_helper.c.
Rename with an "arm_" prefix.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: David Reiss <dreiss@meta.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230227213329.793795-13-richard.henderson@linaro.org
[rth: Split out of a larger patch]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>