Remove the unused argument `Register [dst2/src2]` in [load/store]_sized_value on aarch64 and riscv ports.

Seems like they were brought in in the initial ports JDK-8068054 JDK-8276799 as just straight signature copies of x86. The second register is only required on x86 for x86_32 support and is unused on riscv and aarch64.