[MIPS] Don't crash on unsized extern types with -mgpopt

Summary: This fixes an assertion when building the FreeBSD MIPS64 kernel.

Reviewers: atanasyan, sdardis, emaste

Reviewed By: sdardis

Subscribers: krytarowski, llvm-commits

Differential Revision: https://reviews.llvm.org/D42571

llvm-svn: 323536