runtime: fix invalid nil g check for for mips64x

In CL 292109 we removed unnecessary writes to gp.sched.g
but put wrong register to save g (R4 saves pointer to g) on mips64x

Change-Id: I9777846a7b0a46e1af83dcfc73b74649e0dba3c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/293989


TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Joel Sing <joel@sing.id.au>
Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>