NEGL does a negation of the bottom 32 bits and then zero-extends to 64 bits,
resulting in a negative 32-bit number but a positive 64-bit number.

NEGQ does a full 64-bit negation, so that the result is negative both as
a 32-bit and as a 64-bit number.

This doesn't matter for the functions that are declared to return int32.
It only matters for the ones that return int64 or void* [sic].

This will fix the current incorrect error in the OpenBSD/amd64 build.
The build will still be broken, but it won't report a bogus error.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7536046