target-arm: Don't generate code specific to current CPU mode for SRS

When translating the SRS instruction, handle the "store registers
to stack of current mode" case in the helper function rather than
inline. This means the generated code does not make assumptions
about the current CPU mode which might not be valid when the TB
is executed later.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
