Implement sending signals to blocked-in-kernel processes.

This is dirty but pretty cool! If we have a pending, unmasked signal for
a process that's blocked inside the kernel, we set up alternate stacks
for that process and unblock it to execute the signal handler.

A slightly different return trampoline is used here: since we need to
get back into the kernel, a dedicated syscall is used (sys$sigreturn.)

This restores the TSS contents of the process to the state it was in
while we were originally blocking in the kernel.

NOTE: There's currently only one "kernel resume TSS" so signal nesting
definitely won't work.

Kernel/Syscall.h

@@ -54,6 +54,7 @@
     __ENUMERATE_SYSCALL(umask) \
     __ENUMERATE_SYSCALL(getgroups) \
     __ENUMERATE_SYSCALL(setgroups) \
+    __ENUMERATE_SYSCALL(sigreturn) \
 
 
 #define DO_SYSCALL_A0(function) Syscall::invoke((dword)(function))
@@ -78,6 +79,7 @@ inline constexpr const char* toString(Function function)
     ENUMERATE_SYSCALLS
 #undef __ENUMERATE_SYSCALL
     }
+    return "Unknown";
 }
 
 void initialize();