YJIT: Avoid writing return value to memory in `leave`

Previously, at the end of `leave` we did
`*caller_cfp->sp = return_value`, like the interpreter.
With future changes that leaves the SP field uninitialized for C frames,
this will become problematic. For cases like returning from
`rb_funcall()`, the return value was written above the stack and
never read anyway (callers use the copy in the return register).

Leave the return value in a register at the end of `leave` and have the
code at `cfp->jit_return` decide what to do with it. This avoids the
unnecessary memory write mentioned above. For JIT-to-JIT returns, it goes
through `asm.stack_push()` and benefits from register allocation for
stack temporaries.

Mostly flat on benchmarks, with maybe some marginal speed improvements.

Co-authored-by: Takashi Kokubun <[email protected]>

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