[lldb] Fix offset intersection bug between MPX and AVX registers

Summary:
This change increases the offset of MPX registers (by 128) so they
do not overlap with the offset associated with AVX registers. That was
causing MPX data in GDBRemoteRegisterContext::m_reg_data to get overwritten.

Reviewers: labath

Reviewed By: labath

Subscribers: JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68874