compile and runtime resolving of libraries and entry points for CALL #275

(other compilers have similar options): add config option similar to reserved but to setup "external" names = name_in_cobol:external_name[:call-convention[:[LDFLAGS][:CFLAGS]]] which we would also read for the elements currently in the system definitions.

Note: instead of the call-convention we can already define those including parameters as prototype definitions, then also force-include them with --copy, at least or compile time.
Obviously CFLAGS/LDFLAGS are only relevant for compile-time.

... and there was also the FR to have the dynamic names be configurable in runtime.cfg (a list to map similar as above, but instead of LDFLAGS the so-name to load for the entry point), which would then automatically handle the -fextfh=:name as well.

Originally posted by @GitMensch in #271 (comment)

... just to keep those notes visible