Use 32-bit immediate movslq in macro assembler if 64-bit value fits in 32 bits on x86_64

Currently, Macro::Assembler(Address dst, intprt_t src) on x64 uses an intermediate scratch register to store the 64-bit immediate.

But, if the value fits into 32-bits, we can also explicitly use the 32-bit immediate overload, which saves an instruction and a register use.