Fixes #8976 and #9873 by making use of the Binutils ld.gold
linker explicit whenever the target is linux/ARM or android/ARM.
This does not affect iOS where Apple provides its own linker.

In order to achieve this, we need to add `-fuse-ld=gold` to
the SettingsCCompilerLinkFlags setting and set
SettingsLdCommand to `ld.gold` (or `${target}-ld.gold` when
cross-compiling). In addition, simplifying the use of
`$(CONF_GCC_LINKER_OPTS_STAGEn)`.

This patch was tested by ensuring that the following worked
as expected:

  * Native builds on linux/x86_64 (nothing changed).
  * Native builds on linux/arm (and uses the gold linker).
  * Linux to linux/arm cross compiles (and uses the cross
    gold linker).

Contributions by Ben Gamari, Joachim Breitner and Reid Barton.

Reviewers: nomeata, bgamari, austin, rwbarton

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D715

GHC Trac Issues: #8976 #9873