[ELF] Add REVERSE input section description keyword

The `REVERSE` keyword is described here:

https://sourceware.org/bugzilla/show_bug.cgi?id=27565

It complements `SORT` by allowing the order of input sections to be reversed.

This is particularly useful for order-dependent sections such as .init_array,
where `REVERSE` can be used to either detect static initialization order fiasco
issues or as a mechanism to maintain .ctors element order while transitioning to
the modern .init_array. Such a transition is described here:

https://discourse.llvm.org/t/is-it-possible-to-manually-specify-init-array-order/68649

Differential Revision: https://reviews.llvm.org/D145381

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