The manpages says SA_RESTORER is obsolete, and indeed, not every architecture
support it. However, sadly it's required on x86_64, see http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/signal.c?id=26bcd8b72563b4c54892c4c2a409f6656fb8ae8b#n430, so only use it on x86.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/115450043