Revert "Revert "make vdso function pointers read-only at runtime""

This reverts commit 1946856b.

This goes back to the original scheme of PROT_NONEing a page within
libc. Allocating a new page didn't fail safe for cases where these
functions are called from the dynamic linker.

Bug: http://b/22568628
Change-Id: I3e7241c8b54c27ea4a898bc952375c1e9ae38c80