[LLD][ELF] - Update and fix gnu-ifunc* tests.

It turns that gnu-ifunc-plt-i386.s and gnu-ifunc-plt.s tests are broken.

Initially they were implemented in D27581 and tested that `IRELATIVE` relocations
are placed after other relocations in `.rel.plt`.

Later, we started to place `IRELATIVE` relocations to `.rela.dyn` (D65651).

Also, at some point `.plt` was renamed to `.iplt` (D71520).

Now, `gnu-ifunc*` tests mentioned do not test what they intended to test initially:
they should test that `IRELATIVE` relocations are placed after other ones in
`.rela.dyn`. Also, comments needs to be updated accordingly after changes performed.

This patch updates them.

Differential revision: https://reviews.llvm.org/D85642