syscall, runtime/internal/syscall: zero r2 before mips linux syscalls

syscall, runtime/internal/syscall: zero r2 before mips linux syscalls

All mips variant perform syscalls similarly. R2 (v0) holds r1 and R3
(v1) holds r2 of a syscall. The latter is only used by 2-ret syscalls.
A 1-ret syscall would not touch R3 but keeps it as is, making r2 be a
random value. Always reset it to 0 before SYSCALL to fix the issue.

Fixes #56426

Change-Id: Ie49965c0c3c224c4a895703ac659205cd040ff56
Reviewed-on: https://go-review.googlesource.com/c/go/+/452975
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>