runtime: remove redundant save_g call in mcall for arm

The setg call a few lines earlier has already performed the same iscgo check
and called save_g if necessary.

Change-Id: I6e7c44cef4e0397d6001a3d5b7e334cdfbc3ce22
Reviewed-on: https://go-review.googlesource.com/c/go/+/316929


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>