[ELF] Propagate LMA offset to sections with neither AT() nor AT>

Fixes https://bugs.llvm.org/show_bug.cgi?id=45313
Also fixes linkerscript/{at4.s,overlay.test} LMA address issues exposed by
011b7855.
Related: D74297

This patch improves emulation of GNU ld's heuristics on the difference
between the LMA and the VMA:
https://sourceware.org/binutils/docs/ld/Output-Section-LMA.html#Output-Section-LMA

New test linkerscript/lma-offset.s (based on at4.s) demonstrates some behaviors.

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D76995