[Driver] Don't force .exe suffix for lld

When cross-compiling to Windows using lld, we want the driver to invoke
it as lld-link rather than lld-link.exe. On Windows, the LLVM fs
functions take care of adding the .exe suffix where necessary, so we can
just drop the addition in the toolchain entirely.

Differential Revision: https://reviews.llvm.org/D33923

llvm-svn: 304761