1.0.36.27: less mprotect calls in GENCGC

1.0.36.27: less mprotect calls in GENCGC

 * In unprotect_oldspace(), instead of calling os_protect() for every
   page that needs unprotecting separately, call it once for every
   contiguous region of pages. (Thanks to Vitaly Mayatskikh.)

 * Replace the unprotection calls from free_oldspace() and
   copy_large_object() with asserts: unprotect_oldspace should already
   have unprotected everything of interest.