use HAVE_BUILTIN___BUILTIN_MUL_OVERFLOW

We already check for __builtin_mul_overflow in configure but never
actually referred it before.  Why not call it if available, because
that should render supposedly-optimial assembly outputs.

Optionally if __builtin_mul_overflow_p is available, which is the case
for recent GCC, use that to detect fixnum overflow.  This is much
faster than the previous.  On my machine generated assembly of
numeric.c:int_pow reduces from 480 to 448 bytes, according to nm(1).
Also on my machine, following script boosts from 7.819 to 6.929 sec.

time ./miniruby -e 'i=0; while i < 30_000_000 do i += 1; 7 ** 23; end'

Signed-off-by: Urabe, Shyouhei <shyouhei@ruby-lang.org>


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57784 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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