signed value is undefined

* SunOS linker doesn't support --gc-sections
 * Add portable_endian.h with __sun version of htole64 and le64toh
   * Replace portable endian code in shake.c with inclusion of portable_endian.h
   * Replace portable endian code in word.h with inclusion of portable_endian.h
 * Add explicit extern reference to word.h for posix_memalign when __sun defined
 * Replace references to u_int*_t with uint*_t
 * rdtsc call in shake.c was only working on 32-bit i386
 * rdtsc call in bench_decaf.cxx was inaccurate on 64-bit x86_64 when clang absent
 * Fix two signed/unsigned comparison errors in test_decaf.cxx

Before, invert_elligator would invert to a gf, which wouldnt be a uniformly
random string because, eg, curve25519 gfs only have 255 bits out of 256.
Now add a random multiple of p.  This still wont work for future curves
that have a field size of 1 mod 8, because those curves use elligator with
no high bit set, but its a start