[SOL] Update SBF call ABI

[LucasSte]

Problem

Since commit c981026, the Rust compiler does not allow targets to leak aggregate types to an external ABI. As a consequence, compiling a function extern fn foo(arg: [u8; 16]) for Solana would hit the assertion:

rust/compiler/rustc_codegen_llvm/src/abi.rs

Lines 369 to 374 in 6cc5efc

	assert!(
	matches!(&*cx.tcx.sess.target.arch, "wasm32" | "wasm64")
	|| self.conv == Conv::PtxKernel,
	"`PassMode::Direct` for aggregates only allowed on wasm and `extern \"ptx-kernel\"` fns\nProblematic type: {:#?}",
	arg.layout,
	);

Solution

I updated the SBF's ABI convention to use 64-bit registers when possible and pass arguments/returns indirectly if they are Rust aggregates.