cmd/compile: update TestIntendedInlining for riscv64

Mark nextFreeFast as not inline, as it is too expensive to inline on riscv64.
Also remove riscv64 from non-atomic inline architectures, as we now have
atomic intrisics.

Updates #22239

Change-Id: I6e0e72c1192070e39f065bee486f48df4cc74b35
Reviewed-on: https://go-review.googlesource.com/c/go/+/227808


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>