Rollup merge of #136334 - ricci009:primitivers, r=tgross35

Extract `core::ffi` primitives to a separate (internal) module

### Introduce library/core/src/ffi/primitives.rs

The regex preprocessing for PR #133944 would be more robust if the relevant types from core/src/ffi/mod.rs were first moved to library/core/src/ffi/primitives.rs, then there isn't a need to deal with traits / c_str / va_list / whatever might wind up in that module in the future

r? `@tgross35`