the function libc_errno returns a pointer to a signed-32 bit quantity,
not a 64-bit quantity.

Fixes #27004

Change-Id: I0623835ee34fd9655532251f096022a5accb58cd
Reviewed-on: https://go-review.googlesource.com/129475
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>