[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

lld/ELF/ScriptParser.cpp

@@ -660,6 +660,7 @@ StringMatcher ScriptParser::readFilePatterns() {
 
 SortSectionPolicy ScriptParser::peekSortKind() {
   return StringSwitch<SortSectionPolicy>(peek())
+      .Case("REVERSE", SortSectionPolicy::Reverse)
       .Cases("SORT", "SORT_BY_NAME", SortSectionPolicy::Name)
       .Case("SORT_BY_ALIGNMENT", SortSectionPolicy::Alignment)
       .Case("SORT_BY_INIT_PRIORITY", SortSectionPolicy::Priority)