Fix issue where GPR and FPR registers have overlapping byte offsets.

Summary:
The implementation of GDBRemoteRegisterContext relies on byte offsets to cache
register values. GPR, FPR, etc. should start on different offsets. This is
correctly done in debugserver (in DNBArchImplX86_64.cpp), but not on Linux or
FreeBSD (in RegisterInfos_x86_64.h).

Test Plan: `register read st0` no longer overwrites `rbp` on Linux with LLGS.

Reviewers: sivachandra, jingham, emaste, ovyalov, clayborg

Reviewed By: clayborg

Subscribers: emaste, lldb-commits

Differential Revision: http://reviews.llvm.org/D8685

llvm-svn: 233837