Configure whether the linker supports gc-sections rather than whether it is GNU linker

In compiler/GHC/Linker/Static we check whether the linker is a GNU linker, and proceed to use -Wl,--gc-sections. Instead of configuring whether the linker is GNU into settings, we should configure whether it supports --gc-sections.

NB: During development in !9263 (merged) we came across multiple tasks and bugs regarding the configuration script. In preparation of merging it, we're opening tickets for the leftover tasks both regarding the new ghc-toolchain and the toolchain logic in configure. This is one of those tickets.