[ng] refactor: drop 'HOST', 'BUILD' and 'TARGET' transforms

[ng] refactor: drop 'HOST', 'BUILD' and 'TARGET' transforms

Instead, use the new make variables $(am.conf.host-triplet),
$(am.conf.build-triplet) and $(am.conf.host-triplet) are defined.

* automake.in (preprocess_file): Drop 'HOST', 'BUILD' and 'TARGET'
transforms.  Their role taken over ...
(generate_makefile): ... by the new variables 'am.conf.host-triplet',
'am.conf.build-triplet' and 'am.conf.target-triplet' defined here.
* lib/am/dejagnu.am: Adjust.
* t/triplet.sh: New test.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>