The Go run-time assumes that all SSE floating-point exceptions
are masked so that Go programs are not broken by such invalid
operations. By default, the 64-bit version of the Plan 9 kernel
masks only some SSE floating-point exceptions. Here, we mask
them all on a per-thread basis.

R=rsc, rminnich, minux.ma
CC=golang-dev
https://golang.org/cl/6592056