cmd/asm: remove X27 and S11 register names on riscv64

cmd/asm: remove X27 and S11 register names on riscv64

The X27 register (known as S11 via its ABI name) is the g register on riscv64.
Prevent assembly from referring to it by either of these names.

Change-Id: Iba389eb8e44e097c0142c5b3d92e72bcae8a244a
Reviewed-on: https://go-review.googlesource.com/c/go/+/265519
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>