Since the cause of the problems with the __fillcontextx() was
identified, unify the code of check_deferred_signal() for all
architectures, making the variant under #ifdef x86 common.

Tested by:	marius (sparc64)
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Do not map the shared page COW.  If the process wired its address
space, fork(2) would cause shadowing of the physical object and
copying of the shared page into private copy, effectively preventing
updates for the exported timehands structure and stopping the clock.

Specify the maximum allowed permissions for the page to be read and
execute, preventing write from the user mode.

Reported and tested by:	<huanghwh@yahoo.com>
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks