The hack embodied in commit 4ba61a48 no longer works after today's change
to allow DatumGetFloat4/Float4GetDatum to be inlined (commit 14cca1bf).
Probably what's happening is that the faulty compilers are deciding that
the now-inlined assignment is a no-op and so they're not required to
round to float4 width.

We had a bunch of similar issues earlier this year in the degree-based
trig functions, and eventually settled on using volatile intermediate
variables as the least ugly method of forcing recalcitrant compilers
to do what the C standard says (cf commit 82311bcd).  Let's see if
that method works here.

Discussion: <4640.1472664476@sss.pgh.pa.us>