link/Elf.zig: ensure capacity before appending linker args.

[xxxbxxx]

fixes e567abb "rework linker inputs"

closes #21801

[alexrp]

Fix looks fine in principle.

However, given that we can't really predict how many arguments we're actually going to append, and considering that populating argv is not at all performance-sensitive, I do kind of question the value of using argv.appendAssumeCapacity() here... as opposed to just doing argv.append() like the rest of this entire function is doing, including the -whole-archive handling (which is a bit similar). Just doesn't seem worth the risk of bugs like the one we ran into here. 🤷

(To be clear, I'm referring to the argv.appendAssumeCapacity() calls below, not the one added in this PR.)

[alexrp]

In any case, I've confirmed locally that this fixes the issue, so will merge as-is to unbreak master.