CL 209899 worked around an issue that corrupts vector registers in
recent versions of the Linux kernel by mlocking the top page of every
signal stack on amd64. However, the underlying issue also affects the
XMM registers on 386. This CL applies the mlock fix to both amd64 and
386.

Fixes #35777 (again).

Change-Id: I9886f2dc4c23625421296bd5518d5fd3288bfe48
Reviewed-on: https://go-review.googlesource.com/c/go/+/210345
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>