This removes the signalMap on unix platform, since
the signalMap on different architectures is not same,
especially it's wrong on mipsx.
golang.org/x/sys/unix now has a SignalNum func to convert
signal name to a number, thus there's no need to keep
this redundant map.
Windows platform still needs to have a signalMap, since
golang.org/x/sys/windows doesn't have corresponding
functions.
Address: #3061
Signed-off-by: Shengjing Zhu <zhsj@debian.org>
