Previously, for statically-linked extensions, we used
`vm->loading_table` to delay calling the init function until the
extensions are required. This caused the extensions to look like they
are in the middle of being loaded even before they're required.
(`rb_feature_p()` returned true with a loading path output.) Combined
with autoload, queries like `defined?(CONST)` and `Module#autoload?`
were confused by this and returned nil incorrectly. RubyGems uses
`defined?` to detect if OpenSSL is available and failed when OpenSSL was
available in builds using `--with-static-linked-ext`.

Use a dedicated table for the init functions instead of adding them to
the loading table. This lets us remove some logic from non-EXTSTATIC
builds.

[Bug #19115]