[ELF] De-template LinkerDriver::link. NFC

Replace `f<ELFT>(x)` with `InvokeELFT(f, x)`.
The size reduction comes from turning `link` from 4 specializations into 1.

My x86-64 lld executable is 26KiB smaller.

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D118551