[lld][ELF][test] Fix excessive output file size in loongarch-add-sub.s

Initially the .rodata section came before .text, hence sharing its
segment with the program header sitting at a small offset, pushing the
output file size to ~72GiB (the file was sparse though, so not much is
really written). This breaks on 32-bit platforms and is irrelevant to
the feature being tested, so re-order the two sections so .text gets
processed first, and both sections get their own segment.

This addresses the issue found by the clang-armv8-lld-2stage builder:
    https://lab.llvm.org/buildbot/#/builders/178/builds/5340

Reviewed By: SixWeining, xry111

Differential Revision: https://reviews.llvm.org/D156293