Intel(R) Memory Protection Extensions (Intel(R) MPX) support.

Summary:

The Intel(R) Memory Protection Extensions (Intel(R) MPX) associates pointers
to bounds, against which the software can check memory references to
prevent out of bound memory access.

This patch allows accessing the MPX registers:
  * bnd0-3: 128-bit registers to hold the bound values,
  * bndcfgu, bndstatus: 64-bit configuration registers,

This patch also adds read/write tests for the MPX registers in the register
command tests and adds a new subdirectory for MPX specific tests.

Signed-off-by: Valentina Giusti <valentina.giusti@intel.com>

Reviewers: labath, granata.enrico, lldb-commits, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D24187

llvm-svn: 280668