Correct mprotect page boundries to round up end page. Fixes PR30905.

Summary:
Update the boundries for mprotect.
Patch by Andrew Adams. Fixes PR30905.

Reviewers: loladiro, andrew.w.kaylor, chandlerc

Subscribers: abadams, llvm-commits

Differential Revision: https://reviews.llvm.org/D26312

llvm-svn: 286032