Simplify POSIXThread register context handling

This seems a little more straightforward and is equivalent to r201457
for ELF core files.  A case for FreeBSD i386 is also added (it was
incorrectly using the 64-bit register context and corrupting mememory).

Better (user-facing) error handling is still needed.

Review: http://llvm-reviews.chandlerc.com/D2765