[XRay][compiler-rt] Remove MAP_NORESERVE from XRay allocations

Summary:
This reverses an earlier decision to allow seg-faulting from the
XRay-allocated memory if it turns out that the system cannot provide
physical memory backing that cannot be swapped in/out on Linux.

This addresses http://llvm.org/PR38588.

Reviewers: eizan

Reviewed By: eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50831

llvm-svn: 339869