runtime: tricky replacements of _g_ in signal_unix.go
sighandler has gp, the goroutine running when the signal arrived, and
gsignal, the goroutine executing the signal handler. The latter is
usually mp.gsignal, except in the case noted by the delayedSignal check.
Like previous CLs, cases where the getg() G is used only to access the M
are replaced with direct uses of mp.