amd64: Don't repeat unnecessary tests when cmpset fails

When a cmpset for removing the PG_RW bit in pmap_promote_pde() fails,
there is no need to repeat the alignment, PG_A, and PG_V tests just to
reload the PTE's value. The only bit that we need be concerned with at
this point is PG_M. Use fcmpset instead.

(cherry picked from commit 3687797618b6c978ad733bd206a623e5df47dbe3)