[LLD][ELF][AArch64][ARM] Add missing classof to patch sections.

The code to insert patch section merges them with a comparison function that
uses logic of the form:
return (isa<PatchSection>(a) && !isa<PatchSection>(b));
If the PatchSections don't implement classof this check fails if b is also
a SyntheticSection. This can result in the patches being out of range if
the SyntheticSection is big, for example a ThunkSection with lots of thunks.

Differential Revision: https://reviews.llvm.org/D71242

fixes (part of) pr44071

(cherry picked from commit 247b2ce1)