cwrappers: Add cross-compilation support.

- New option `sysroot=<dir>':
  . Wrapper will add `--sysroot=<dir>' as first argument.
  . For every rpath argument, e.g. -Wl,-R<path> to cc, wrapper will
    pass `-rpath-link <dir><path>' to the linker.
  This matches the old mk/wrapper/cmd-sink-cross-* logic.

- Create wrappers for the ${MACHINE_GNU_PLATFORM}-cc style of command,
  as in ${CC}, ${CXX}, ${LD} for cross-builds.

- Bump version.

- Use TOOL_DEPENDS, not BUILD_DEPENDS, for cwrappers.

(riastradh)