Complete Mauke's work on __builtin_*_overflow for faster overflow checks?

[richardleach]

Description
Lukas Mai worked on adding support for using compiler intrinsics (e.g. __builtin*overflow) for overflow checks where available, but this hasn't yet been completed. As suggested on IRC, this issue is a placeholder/reminder, in case anyone fancies picking this up in the future.

Correspondence is at https://www.nntp.perl.org/group/perl.perl5.porters/2017/10/msg246823.html
A smoke-me branch is at smoke-me/mauke/overflow

Summarizing the apparent last status:

• Dave Mitchell reported that (on his hardware at the time) that patches resulted in a modest improvement where intrinsics could be used, with no slowdowns where intrinsics could not be used.
• There were problems with number conversion (e.g. failures in t/op/numconvert.t) to investigate.