Skip building x86 parts of UnwindRegisters*.S when targeting SjLj

This extends SVN r314197 from the arm parts to the whole file.

Differential Revision: https://reviews.llvm.org/D38381

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@314492 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed