ftruncate.
* ftruncate.c: New file, from gnulib.
* output.cc (posix_fallocate): Define dummy version if not
HAVE_POSIX_FALLOCATE.
(Output_file::map): Call posix_fallocate rather than lseek and
write.
* gold.h (ftruncate): Declare if not HAVE_FTRUNCATE.
* configure, Makefile.in, config.in: Rebuild.