CL 203284 added a compiler intrinsics from atomic Load8 and Store8 on
several architectures, but missed the lowering on MIPS. This CL fixes
that.

Updates #10958, #24543.

Change-Id: I82e88971554fe8c33ad2bf195a633c44b9ac4cf7
Reviewed-on: https://go-review.googlesource.com/c/go/+/203977
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>