[ELF] Make compareByFilePosition a strict weak order

This fixes the new test linkerscript/enable-non-contiguous-regions.test
from #90007 in -stdlib=libc++ -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG builds.

adjustOutputSections does not discard the output section .potential_a
because it contained .a (which would be spilled to .actual_a).
.potential_a and .bc have the same address and will cause an assertion
failure.