Port cksum builtin performance improvements from illumos

[JohnoKing]

This commit ports performance optimizations from illumos for the libsum code (used by the cksum and sum builtins):
illumos@98bea71
The new codepath in libsum uses prefetching and loop unrolling to improve performance (prefetching is done with __builtin_prefetch() or sun_prefetch_read_many() if either is available).

Script for testing (note that cksum must be enabled in src/cmd/ksh93/data/builtins.c):

$ cat /tmp/foo
#!/bin/ksh
builtin cksum || exit 1
for ((i=0; i!=50000; i++)) do
    cksum -x att /etc/hosts
done >/dev/null

Results on Linux x86_64 (using CCFLAGS=-O2):

$ echo 'UNPATCHED:'; time arch/linux.i386-64/bin/ksh /tmp/foo; echo 'PATCHED'; time /tmp/ksh /tmp/foo
UNPATCHED:

real	0m09.989s
user	0m07.582s
sys	0m02.406s
PATCHED:

real	0m06.536s
user	0m04.331s
sys	0m02.204s

src/lib/libsum/{sum-att.c,sum-crc.c,Mamfile}:
- Port the performance optimizations from illumos to 93u+m libsum. To prevent problems with older versions of GCC, avoid the new codepath if GCC is older than the 3.1 release series. Additionally, the ast.h header must be included to handle tcc defining __GNUC__ on FreeBSD.
- Apply some build fixes to allow the new codepath to build with Clang 3.6 and newer (my own testing indicates an even better performance improvement with Clang than with GCC).