S_uiv_2buf: faster integer stringification algorithm

S_uiv_2buf: faster integer stringification algorithm

Processing two digits at a time results in noticeable performance
improvement for larger numbers.

The previous version of the function was being automatically inlined
by gcc because of its small size. It's no longer the case so I had to
use PERL_STATIC_INLINE.

I have marked the UV branch as UNLIKELY because UVs are *much* rarer
than IVs.

[perl #133691]