Revert r211866, r211895 and r211995, "Driver: use GNU::Link for the Generic_GCC toolchain". It broke users of Generic_GCC, cygwin and mingw32.

It reverts commits as follows:
  r211866: "Driver: use GNU::Link for the Generic_GCC toolchain"
  r211895: "Replace GetProgramPath("ld") with GetLinkerPath()."
  r211995: "Driver: add a cygwin linker tool"

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211998 91177308-0d34-0410-b5e6-96231b3b80d8