Remove an x86-ism from RegisterInfoInterface

Summary:
While adding FPR support to x86 elf core files (D26300), we ended up adding a
very x86-specific function to the general RegisterInfoInterface class, which I
didn't catch in review. This removes that function. The only reason we needed
it was to find the offset of the FXSAVE area. This is the same as the offset of
the first register within that area, so we might as well use that.

Reviewers: clayborg, dvlahovski

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D27222

llvm-svn: 288236