elf-core: support 32- and 64-bit x86 registers

This way the same RegisterContext class can support i386 and
amd64/x86_64 core files.

With some further refinement we should be able to merge all of the
processor-specific RegisterContextPOSIX_* classes into a single shared
one.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201577 91177308-0d34-0410-b5e6-96231b3b80d8