Missing extern "C" for ffi functions

[bjorn3]

By default rust functions use the rust calling convention which is unstable and may not match the C calling convention that is likely expected by the Pharo VM. You need to use extern "C" fn to use the C calling convention.

gtoolkit-vm/vm-runtime/src/virtual_machine.rs

Lines 177 to 178 in 0733778

	#[no_mangle]
	pub fn is_virtual_machine() {

and elsewhere.