x86_64/i386 and illumos/solaris/SunOS compatibility fixes.
* 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