[ELF] Support -r --gc-sections

-r --gc-sections is usually not useful because it just makes intermediate output
smaller. https://bugs.llvm.org/show_bug.cgi?id=46700#c7 mentions a use case:
validating the absence of undefined symbols ealier than in the final link.

After D84129 (SHT_GROUP support in -r links), we can support -r
--gc-sections without extra code. So let's allow it.

Reviewed By: grimar, jhenderson

Differential Revision: https://reviews.llvm.org/D84131