[M68k] Override `CanLowerReturn` to fix assertion with large return

If it couldn't fit the return value in two registers, this caused an
error during codegen. It seems this method is implemented in other
backends but not here, and allows it to pass return values in memory
when it isn't able to do so in registers.

Seems to fix compilation of Rust code with certain return types:
https://github.com/rust-lang/rust/issues/89498

Differential Revision: https://reviews.llvm.org/D148856