[lld][mips] Correct tests for mips64 implying PIC.

Currently LLVM can only generate PIC code for MIPS64 with the N64 as
it uses the idiom "isPositionIndependent() || IsABI_N64()" throughout the
MIPS backend. r293164 changed this, causing test failures for LLD.

This patch changes the tests minimally to preserve existing test coverage
and one case where the test was "right" in the wrong circumstance.

Reviewers: atanasyan

Differential Revision: https://reviews.llvm.org/D29194

llvm-svn: 293275