stdalign: port to HP-UX compilers

* lib/stdalign.in.h (_Alignas): Use __attribute__ (__aligned__ (x))
if __HP_cc or __HP_aCC are nonzero.
fdopen-tests: port to Tru64

* tests/test-fdopen.c (main): Don't invoke fdopen on a file
descriptors that is not open, as POSIX doesn't specify the
resulting behavior and the test does not work on Tru64.
Problem reported by Steven M. Schweda in:
http://lists.gnu.org/archive/html/bug-gnulib/2014-01/msg00079.html
Fix typo in ChangeLog.