cmd/compile: don't depend on arch-dependent conversions in the compiler

cmd/compile: don't depend on arch-dependent conversions in the compiler

Leave those constant foldings for runtime, similar to how we do it
for NaN generation.

These are the only instances I could find in cmd/compile/..., using

objdump -d ../pkg/tool/darwin_arm64/compile| egrep "(fcvtz|>:)" | grep -B1 fcvt

(There are instances in other places, like runtime and reflect, but I don't
think those places would affect compiler output.)

Change-Id: I4113fe4570115e4765825cf442cb1fde97cf2f27
Reviewed-on: https://go-review.googlesource.com/c/go/+/711281
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>