runtime: zero upper bit of Y registers in asyncPreempt on darwin/amd64

Apparently, the signal handling code path in darwin kernel leaves
the upper bits of Y registers in a dirty state, which causes many
SSE operations (128-bit and narrower) become much slower. Clear
the upper bits to get to a clean state.

We do it at the entry of asyncPreempt, which is immediately
following exiting from the kernel's signal handling code, if we
actually injected a call. It does not cover other exits where we
don't inject a call, e.g. failed preemption, profiling signal, or
other async signals. But it does cover an important use case of
async signals, preempting a tight numerical loop, which we
introduced in this cycle.

Running the benchmark in issue #37174:

name    old time/op  new time/op  delta
Fast-8  90.0ns ± 1%  46.8ns ± 3%  -47.97%  (p=0.000 n=10+10)
Slow-8   188ns ± 5%    49ns ± 1%  -73.82%  (p=0.000 n=10+9)

There is no more slowdown due to preemption signals.

For #37174.

Change-Id: I8b83d083fade1cabbda09b4bc25ccbadafaf7605
Reviewed-on: https://go-review.googlesource.com/c/go/+/219131
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
2 files changed
tree: 87c73b63b74568419b164d64b4f5509ffaad9aae
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