alpha-linux-user: Implement signals.

Move userland PALcode handling into linux-user main loop so that
we can send signals from there.  This also makes alpha_palcode.c
system-level only, so don't build it for userland.  Add defines
for GENTRAP PALcall mapping to signals.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
