configure: Coerce gcc to use $LD instead of system default

The configure script will now try to coerce gcc to use the linker
pointed to by $LD instead of the system default (typically bfd ld).
Moreover, we now check for `ld.gold` and `ld.lld` before trying `ld`.

The previous behavior can be reverted to by using the new
--disable-ld-override flag.

On my machine gold seems to trigger an apparent infelicity in
constructor behavior, causing T5435_asm to fail. I've opened #13883 to
record this issue and have accepted the questionable constructor
ordering for the time being.

Test Plan: Validate with `config_args='--enable-ld-override'`

Reviewers: austin, hvr, simonmar

Subscribers: duog, nh2, rwbarton, thomie, erikd, snowleopard

GHC Trac Issues: #13541, #13810, #13883

Differential Revision: https://phabricator.haskell.org/D3449