Use OQS_*_set_callbacks instead of C_OR_NI_OR_ARM macros

[SWilson4]

          This isn't something to change in this PR, but something I noticed just from looking at this code during review.  Now that we have the ability to set callbacks, could we get rid of the `C_OR_NI_OR_ARM` macros and instead set the callbacks once (either during library init, or alternatively on first use if it's not set)?  Again: not something to do in this PR, but if anyone else thinks it might be good, we could create a separate issue to track.

Originally posted by @dstebila in #1707 (comment)

See also: #1599, #1832