Prefer $NM @file when the toolchain isn't native to $build.

Prefer $NM @file when the toolchain isn't native to $build.

* libltdl/config/ltmain.m4sh (func_mode_link): Avoid calculating
the command line length and take the @file branch *if* the file
names needs to be converted for the toolchain and the @file
branch works.  Fixes problems in stresstest.at when doing a cross
from Cygwin to MinGW using MinGW native tools (a.k.a. a "faked"
cross), and for the specific case of MSVC on Cygwin it makes the
test pass.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>