[Hexagon] Mark raise_relocation_error as NORETURN.

Summary:
This silences a couple of implicit fallthrough warnings with GCC 7.1 in
this file.

Reviewers: colinl, kparzysz

Reviewed By: kparzysz

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35889

llvm-svn: 309129