Fix linux x86 debugging on a linux x86 host (32-bit on 32-bit).

This change fixes up issues with specifying the size of the i386
register infos for FPU registers.  The bug was that for the i386
register context, the size of the FPU registers were still being
computed based on the x86_64 FXSAVE structure.

This change permits the FPR_SIZE macro to optionally be defined
outside of RegisterInfos_i386.h, which RegisterContextLinux_i386.cpp
does properly. It redefines the FPR_i386 structure with all the
accessible parts that RegisterInfos_i386.h wants to see, which we had
not done before when we made the overall size of the structure
properly sized a recently.

This change also modifies POSIXThread to create a
RegisterContextLinux_i386 only when the host is 32-bit; otherwise, it
uses the RegisterContextLinux_x86_64, which works properly for 32-bit
and 64-bit inferiors on a 64-bit host.

I tested this debugging a Linux x86 exe on an x86 host (Ubuntu 13.10
x86), and debugging a Linux x86 exe and a Linux x86-64 exe on an
x86-64 host (Ubuntu 12.04 LTS).  Those cases all worked.

Thanks to Matthew Gardiner who discoverd may key insights into
tracking down the issue. The motivation for this change and some of
the code originates from him via this thread:

http://lists.cs.uiuc.edu/pipermail/lldb-commits/Week-of-Mon-20140224/010554.html