Kernel: Userspace signal handlers are now called one at a time

I added a syscall for telling the kernel when signal execution has
finished. We should send a random hash or id to the signal trampoline
that we would include in the syscall, so validity of signal exit can
be confirmed.

kernel/CMakeLists.txt

@@ -46,6 +46,7 @@ set(KERNEL_SOURCES
 	kernel/Scheduler.cpp
 	kernel/Semaphore.cpp
 	kernel/Serial.cpp
+	kernel/Signal.cpp
 	kernel/SpinLock.cpp
 	kernel/SSP.cpp
 	kernel/Storage/ATABus.cpp