Support platforms where strtoll/strtoull are spelled __strtoll/__strtoull.

Ancient HPUX, for one, does this.  We hadn't noticed due to the lack
of regression tests that required a working strtoll.

(I was slightly tempted to remove the other historical spelling,
strto[u]q, since it seems we have no buildfarm members testing that case.
But I refrained.)

Discussion: https://postgr.es/m/151935568942.1461.14623890240535309745@wrigleys.postgresql.org