kernel: add optimized versions of PROD_INTOBJS

[fingolfin]

Add three variants of PROD_INTOBJS using portable GCC extensions such as
__builtin_smul_overflow (supported by GCC >= 5.0 and clang >= 3.4)

I took great care to verify via test programs written in that the new
implementations yields identical results to PROD_INTOBJS, at least on
64bit systems (and I see no reason why 32bit should be different).

Regarding performance: in a C micro benchmark, the new code outperforms
the old by a factor of 2 on my laptop. In GAP, the gain is not as
impressive, probably because other things add a lot of overhead. Still,
there is a gain of about 10%-15% with this micro benchmark:

foo:=function(b)
local r,i,j,x;
r:=[-b..b];;
for i in r do
for j in r do
x:=i*j;
od;
od;
return x;
end;

Old code:
gap> foo(2^12);;time;
1480
gap> foo(2^13);;time;
5559

New code:
gap> foo(2^12);;time;
1186
gap> foo(2^13);;time;
4849

For comparison, the C mini benchmark does something very similar, but is
faster by a factor 18 (for inputs 2^12, 2^13) to 40 (for input 2^14).

Disassembling the code generated by GCC and clang for the new
implementation shows that it is very close what I'd write by hand, while
the old code is a very complicated, and requires divisions in some cases
(= slow).

Please make sure that this pull request:

• is submitted to the correct branch (the stable branch is only for bugfixes)
• contains an accurate description of changes for the release notes below
• provides new tests or relies on existing ones
• correctly refers to other issues and related pull requests

Tick all what applies to this pull request

• Adds new features
• Improves and extends functionality
• Fixes bugs that could lead to crashes
• Fixes bugs that could lead to incorrect results
• Fixes bugs that could lead to break loops

Write below the description of changes (for the release notes)

[codecov-io]

Current coverage is 49.72% (diff: 100%)

Merging #1047 into master will increase coverage by 0.06%

@@             master      #1047   diff @@
==========================================
  Files           424        424          
  Lines        223275     223191    -84   
  Methods        3429       3429          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits         110880     110986   +106   
+ Misses       112395     112205   -190   
  Partials          0          0          

Powered by Codecov. Last update fd9d4e3...f96b61d

[ChrisJefferson]

This looks fine to me. I'll count on your changes to the autoconf being right, they look "right enough" to me. Will just see if anyone else has a comment.

[fingolfin]

I just slightly adjusted the autoconf code, to make it more flexible, as I want to use it in the future to check for other builtins which require a different number of arguments.

And yeah, I am pretty sure they are "right enough" ;-).

[fingolfin]

Wouldn't a rebase have been nice for this? This, together with the merge of #960, now makes for a somewhat confusing history again. Alternatively, if you prefer a merge commit, then I'd have been happy to rebase this first, before the merge, to make for simpler history.

[ChrisJefferson]

I just forget all the options of github.