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We restored registers correctly in the usual case where the thread
is a Go-managed thread and called runtime·sighandler, but we
failed to do so when runtime·sigtramp was called on a cgo-created
thread. In that case, runtime·sigtramp called runtime·badsignal,
a Go function, and did not restore registers after it returned

LGTM=rsc, dave
R=rsc, dave
CC=golang-codereviews, minux.ma
https://golang.org/cl/105280050