[GR-75099] Hide C API helpers.

[graalvmbot]

This change reduces the GraalPy-specific ABI surface. In particular, we exported all our internal helper/upcall functions and variables.

The new policy for a symbol being exported is:

1. They are part of the public Python API,
2. necessary to implement macros (e.g. GraalPyFloat_AS_DOUBLE), or
3. any of GraalPy_CAPI_HELPERS, GraalPy_CAPI_METADATA, and initialize_graal_capi.

Everything else should be hidden or local.

Major changes summary:

• Since we were using some of our internal C API functions in GraalPy-specific tests (which build a separate module), I've introduced header graalpy/testcapi.h which defines a struct with the necessary members. The instance of this struct is then exposed via a capsule. This is the Python way to do it.
• For the C functions we call from Java, I've introduced a function pointer table (C global variable GraalPy_CAPI_HELPERS) that is generated from NativeCAPISymbol. In that case, it doesn't matter if the symbol is exported or hidden.
• I've introduced CPP macro GraalPy_CAPI_HELPER_SYMBOL to mark such helper functions. The macro is just an alias for Py_LOCAL_SYMBOL.
• I also took the opportunity and merged the offsets and struct sizes (declared by CFields and CStructs) into table, i.e., GraalPy_CAPI_METADATA.
• The initialization of the Java-side mirror of those tables is now actually independent from C API initialization. Also, we don't need to call anything, we just read off-heap memory. This should make C API int a bit faster (but probably not measurable). However, currently the two tables are initialized together with the C API since they will only then be used.
• I've also updated files abi/abi-graalpy250.xml and abi/suppressions-base.ini.
  We do no longer suppress GraalPyPrivate-prefixed symbols.