linux-user/main.c: Use TARGET_SIG* instead of SIG*

In main.c, all SIG* should be TARGET_SIG*, since the relevant functions
(queue_signal() and gdb_handlesig()) expect TARGET_SIG*.

The corresponding vi command is "1,$ s/\<SIG/TARGET_SIG/g".

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>