Don't use the GNU make function 'realpath' (reverting e66a81c6),
because it doesn't handle spaces in paths. Instead, use 'cygpath' to
convert Windows style paths (e.g. C:/foo/) to mingw style paths (e.g.
c/foo/), and then call 'which'.

Tests:
 * make TEST_HC=ghc
 * make TEST_HC=/d/home/thomie/ghc-validate/bindisttest/install\ \ \ dir/bin/ghc
 * make TEST_HC=D:/home/thomie/ghc-validate/bindisttest/install\ \ \ dir/bin/ghc

Reviewed by: Phyx

Differential Revision: https://phabricator.haskell.org/D1431