UAPI: move extern C to dediacted ffi_c module

[pthierry-ledger]

The goal of this PR is to avoid any extern C usage when using full Rust userspace, moving the C FFI support to a dedicated module.
The following paradigm is defined:

• In Rust, the syscall module is to be used by upper layer (typically shield). This module is safe downto arch-specific syscall implementation
• In C the ffi_c module is to be used and hold the __sys prefix for each syscall, as this API is bare and, like for Rust, should be used through the shield abstracted API

cargo-fmt has been fixed for overall UAPI
cargo-clippy is fixed in all UAPI except svc_exchange (to be updated in another PR)

NOTE: this PR impact the kernel API