configure : LLVM and LD detections improvements (#10234).

* distrib/configure.ac.in : Detect correct version of LLVM tools as
  was done for the top level configure.ac in 42448e37.
* aclocal.m4 : Add a FIND_LD macro that can be used in both
  configure scripts.
* Use new FIND_LD macro in both configure scripts.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>

Test Plan: validate on amd64-linux and armhf-linux.

Reviewers: rwbarton, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D828

GHC Trac Issues: #10234