embed.pl: Fix BBC build failures #24196, #24197

[khwilliamson]

Fixes #24196
Fixes #24197

This bug stemmed from a C preprocessor nuance that I was unaware of. It
only happened on unthreaded builds and only affected functions that have
a single argument, the thread context, so that on unthreaded builds,
these have 0 parameters. The preprocessor dropped the parens indicating
there is an argument list (empty), which led to compilation errors.
It does not do this if there is at least one argument besides the thread
context one.

The solution is to extend the use of a paradigm already in effect for
other situations, so that there is a placeholder argument for the thread
context that gets dropped. This is only defined on unthreaded builds.
It is cleaner to do this for unaffected functions as well, (those that
have an argument besides the thread context).

Threaded is unchanged.

• This set of changes does not require a perldelta entry.

[jkeenan]

I installed a local branch derived from this p.r. and built an unthreaded perl on Linux. I was then able to install Sys::Ibam and Wifi::WpaCtrl against that perl.

[gh-24200-khwilliamson-fix_longs-20260216] 2020 $ ./bin/perl -Ilib -MSys::Ibam -MWifi::WpaCtrl -E 'say join "|" => ($Sys::Ibam::VERSION, $Wifi::WpaCtrl::VERSION, "hello world");'
0.05|0.02|hello world

So in terms of the original complaint, this DWIMs.

perlguts code should get additional eyeballs.