crypto/x509, runtime: fix occasional spurious “certificate is expired”

As documented in #51209, we have been seeing a low-rate failure
on macOS builders caused by spurious x509 “certificate is expired” errors.

The root cause is that CFDateCreate takes a float64, but it is being
passed a uintptr instead. That is, we're not even putting CFDateCreate's
argument in the right register during the call. Luckily, having just
computed the argument by calling time.Duration.Seconds, which
returns a float64, most of the time the argument we want is still
in the right floating point register, somewhat accidentally.

The only time the lucky accident doesn't happen is when the goroutine
is rescheduled between calling time.Duration.Seconds and calling
into CFDateCreate *and* the rescheduling smashes the floating point
register, which can happen during various block memory moves,
since the floating point registers are also the SIMD registers.

Passing the float64 through explicitly eliminates the problem.
It is difficult to write a test for this that is suitable for inclusion
in the standard library. We will have to rely on the builders to
start flaking again if somehow this problem is reintroduced.

For future reference, there is a standalone test that used to fail
every few seconds at https://go.dev/play/p/OWfDpxgnW9g.

Fixes #51209.

Change-Id: I8b334a51e41f406b13f37270e9175c64fe6f55ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/387255


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>