[llvm-objdump] Fix unstable disassembly output for sections with same address

When two sections shared the same address, the disassembly code was
using pointer values when sorting (see the SectionRef less than
operator). Since those values aren't guaranteed to have a specific
order, this meant the disassembly code would sometimes change which
section to pick when finding symbols targeted by calls in fully linked
objects.

This change fixes the non-determinism, so that the same section is
always picked. This might have a negative impact in that now a section
without any symbol might be picked over a section with symbols, but this
will be addressed in a later commit.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45411.

Reviewed by: grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D77640