posix: sysconf: add a "small" implementation option

Previously, sysconf() was only available as a macro (i.e. the
"extra-small" option).

This has the advantage of being compile-time constant, and
optimized for both space and speed. One disadvantage is that
querying an `_SC_` value that was invalid or unsupported
would result in a compile error.

Provide a "small" implementation of sysconf() (via Kconfig
choice) as a normal addressable function.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>