dev: winch(x64): Derive the operand size for `masm.zero` (#9292)

This commit derives the operand size from `ABI::word_bytes()` in order
to zero the given register rather than hardcoding it as a 32-bit size
operand.

The motivation for this commit is mainly avoiding hardcoding operand
sizes in Winch's emission layer where possible, especially in the
MacroAssembler.