Don't attempt to use mpx registers on unsupported platforms

Summary:
The existing cpp-level checks using PR_MPX_ENABLE_MANAGEMENT aren't sufficient,
as this isn't defined for linux kernel versions below 3.19.

Reviewers: valentinagiusti, zturner, labath

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D32719

llvm-svn: 302027