Build shared library directly

[emmatyping]

This commit modifies the build system to directly generate shared objects rather than generating a static library that is linked to libpython into a shared object. This removes the link hack that we currently use.

The downside is that we should probably figure out a way to handle custom linker flags and customization of BLDSHARED.

[emmatyping]

Hmm CI still isn't running...

[Eclips4]

We should remove #[unsafe(no_mangle)] from _base64_clear & _base64_free & standard_b64encode to make Check if generated files up are up to date pass (it's basically check's whether exported symbols starts with Py prefix).
I guess these symbols are not expected to be exported?

[Eclips4]

macOS failures are unrelated to this PR, I've seen them yesterday on our main branch, I'll try to figure out how to fix them

[emmatyping]

macOS/Android/iOS all seem to be failing at symbols being undefined. I wonder if the way I am pulling in libpython.a isn't quite right there.

[emmatyping]

Ah I should probably also handle --enable-shared and --without-static-libpython https://docs.python.org/3/using/configure.html#id13

[emmatyping]

Thinking about this more, I don't think this will work out :(

Since we link modules statically and dynamically depending on the platform and build configuration, I think we'll need to re-evaluate a path forward here.