cmd/compile: skip runtime.nextFreeFast inlining test on MIPS64x

cmd/compile: skip runtime.nextFreeFast inlining test on MIPS64x

Since inlining budget calculation is fixed in CL 70151
runtime.nextFreeFast is no longer inlineable on MIPS64x because
it does not support Ctz64 as intrinsic. Skip the test.

Updates #22239.

Change-Id: Id00d55628ddb4b48d27aebfa10377a896765d569
Reviewed-on: https://go-review.googlesource.com/72271
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>