Now that access to the page's "act_count" field is synchronized by the page
lock instead of the object lock, there is no reason for vm_page_activate()
to assert that the object is locked for either read or write access.
(The "VPO_UNMANAGED" flag never changes after page allocation.)

Sponsored by:	EMC / Isilon Storage Division
Move <stdatomic.h> into sys/sys/.

This will allow us to use C11 atomics in kernelspace, although it will
need to be included as <sys/stdatomic.h>.