Re-enable TestFP in cmd/compile/internal/gc on mips64.

Fixes #15552.

Change-Id: I5c3a5564b94d28c723358f0862468fb6da371991
Reviewed-on: https://go-review.googlesource.com/22835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>