COFF: Fix symbols being incorrectly sorted within section groups

[LagoLunatic]

objdiff sorts all symbols when reading the object (#316), but this currently only sorts them within their own section. COFF has groups of sections like rdata and multiple rdata$rs. This PR fixes the sorting so the symbols are sorted relative to other sections within the same group first. This fixes two bugs:

• The combine sections feature would change the order of symbols when enabled (fixesSection merging seems to alter section order #392)
• The order diffing feature incorrectly marks some symbols in grouped sections as being in the wrong order, even when they're in the right order and sections are unmerged