Port cksum builtin performance improvements from illumos (#391)

This commit ports performance optimizations from illumos for the libsum
code (used by the cksum and sum builtins):
98bea71f0d
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):
   #!/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).

NEWS

@@ -14,6 +14,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   Note that to use these options the operating system must support the
   corresponding resource limit.
 
+- Ported performance optimizations from illumos to improve the performance
+  of the cksum builtin. (Note that the cksum builtin is not enabled by
+  default.)
+
 2021-12-27:
 
 - Two bash-like flags for 'whence' were backported from ksh 93v-: