runtime: remove VZEROUPPER patch once Darwin <10.15.6 is not supported

[cuonglm]

Split out from #37174 (comment)

So once we stop supporting releases <10.15.6, we can get rid of the VZEROUPPER patch.

cc @randall77 @dmitshur

[dmitshur]

Apple has been making a new major macOS version each year, and Go has been dropping support for old macOS versions at the same rate. So we can estimate.

Go 1.11 started to require macOS 10.10 or later.
Go 1.13 started to require macOS 10.11 or later.
Go 1.15 started to require macOS 10.12 or later.
If that rate doesn't change, Go 1.23 will start to require macOS 10.16 (aka macOS 11.0) or later, and this optimization can be applied then.

[HenkPoley]

Potentially this issue was reintroduced (elsewhere?) in macOS 10.15.7 (19H15?): https://twitter.com/Cyan4973/status/1327023284974538754

[randall77]

For future self, make sure the reproducers in #37174 perform ok before removing the VZEROUPPER.

[randall77]

Just upgraded to 10.15.7 and it looks still fixed to me.
(The C/asm reproducer runs at the same speed regardless of whether the vzeroupper is commented out or not.)

[mdempsky]

According to https://golang.org/doc/go1.16, Go 1.17 will still support macOS 10.13. So should this be deferred to Go 1.18 (or Go 1.23, per @dmitshur)?

[dmitshur]

I understand this isn't so critical that we need to re-evaluate and bump every 6 months, so I'll put this in a future Go 1.23 milestone. If something changes before then, we can revisit this.