Write protect executable pages in the VM.

Write protect executable pages in the VM.

Change executable pages to be read/execute but not writable by default.

All pages are made temporarily writable just before a full GC, because both
the mark and sweep phases write to the pages.  When allocating in a page and
when patching code, the pages are made temporarily writable.

The order of allocation of Code and Instructions objects is changed so that
a GC will not occur after Instructions is allocated.  (A full GC would
render the Instructions unwritable.)  A scoped object is used to make memory
protection simpler.