Revert r211866, r211895 and r211995, "Driver: use GNU::Link for the...

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"

llvm-svn: 211998