xen/arm32: Use alternative to skip the check of pending serrors

xen/arm32: Use alternative to skip the check of pending serrors

We have provided an option to administrator to determine how to
handle the SErrors. In order to skip the check of pending SError,
in conventional way, we have to read the option every time before
we try to check the pending SError. This will add overhead to check
the option at every trap.

The ARM32 supports the alternative patching feature. We can use an
ALTERNATIVE to avoid checking option at every trap. We added a new
cpufeature named "SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT". This feature
will be enabled when the option is not diverse.

Signed-off-by: Wei Chen <Wei.Chen@arm.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>