Solaris 11.4 has built-in functions named b64_encode and b64_decode.
Rename ours to something else to avoid the conflict (fortunately,
ours are static so the impact is limited).

One could wish for less duplication of code in this area, but that
would be a larger patch and not very suitable for back-patching.
Since this is a portability fix, we want to put it into all supported
branches.

Report and initial patch by Rainer Orth, reviewed and adjusted a bit
by Michael Paquier

Discussion: https://postgr.es/m/ydd372wk28h.fsf@CeBiTec.Uni-Bielefeld.DE