rdar://problem/11487457

Add convenience registers eax, ebx, ecx, edx, edi, esi, ebp, esp to the 'register read' command for x86_64.
Add a GDBRemoteRegisterContext::Addx86_64ConvenienceRegisters() method called from ProcessGDBRemote::BuildDynamicRegisterInfo().
Servicing of eax, for example, is accomplished by delegating to rax with an adjusted offset into the register context.

llvm-svn: 157230