Improved support for LDC, incl. cross-compilation

The ldc2 driver now supports
* some more command-line options (coverage, profiling, keeping stack frame),
* separate linking,
* cross-compilation by specifying the target triple (LDC `-mtriple`) as `--arch`, e.g., `--arch=x86_64-pc-windows-msvc`.
  Check out https://wiki.dlang.org/Cross-compiling_with_LDC for how to setup LDC.