[COFF] [X86] Don't use llvm_unreachable for unsupported relocation types

This can happen if assembling a reference to _GLOBAL_OFFSET_TABLE_.

While it doesn't make sense to try to assemble that for COFF,
the fact that we previously used llvm_unreachable meant that the code
had undefined behaviour if something tried to assemble that.

The configure script of libgmp would try to assemble such a snippet
(which should signal a failure). If llvm is built without assertions,
the undefined behaviour meant a (near) infinite loop.

Differential Revision: https://reviews.llvm.org/D52903

llvm-svn: 343811