syscall: fix riscv64 syscall assembly

syscall: fix riscv64 syscall assembly

RISCV has no instruction to store an immediate value to memory, or to
subtract an immediate value. An immediate needs to be moved to a register
first, or in the case of $0, the zero value register (ZERO or R0) can be
used instead. Restore the original riscv-go code so that it compiles.

Updates #27532

Change-Id: I3c60e6f385c59679252a710dbba153357a367516
Reviewed-on: https://go-review.googlesource.com/c/go/+/212761
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>