[Patch for Review] Merge RegisterContextPOSIX_x86_64 and RegisterContextPOSIX_i386 into RegisterContextPOSIX_x86

All code (at the moment) between RegisterContextPOSIX_i386 and RegisterContextPOSIX_x86_64 is now shared and uses just register offsets, so this checkin merges the two into a single RegisterContextPOSIX_x86. This should (hopefully) help Ed get the 32-bit version of FreeBSD lldb a bit further along as well.

I’ve run this through the 64-bit and -A i386 linux test suite.

Let me know if this looks ok. Thanks.

I will let the linux and FreeBSD folks chime in on this one. Looks good as far as I can tell.