[release-branch.go1] runtime: fix struct Sigaction for Linux/ARM

[release-branch.go1] runtime: fix struct Sigaction for Linux/ARM

««« backport 6eefce99091e
runtime: fix struct Sigaction for Linux/ARM
        if we were to use sizeof(sa.sa_mask) instead of 8 as the last argument
        to rt_sigaction, we would have already fixed this bug, so also updated
        Linux/386 and Linux/amd64 files to use that; also test the return value
        of rt_sigaction.

R=dave, rsc
CC=golang-dev
https://golang.org/cl/6297087

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