+;;; Note on the SYNCHRONOUS argument: On builds without pseudo-atomic,
+;;; we have no way of knowing whether interrupted code was in an
+;;; allocation sequence, and cannot delay signals until after
+;;; allocation. Any signal that can occur asynchronously must be
+;;; considered unsafe for immediate execution, and the invocation of its
+;;; lisp handler will get delayed into a newly spawned signal handler
+;;; thread. However, there are signals which we must handle
+;;; immediately, because they occur synchonously (hence the boolean flag
+;;; SYNCHRONOUS to this function), luckily implying that the signal
+;;; happens only in specific places (illegal instructions, floating
+;;; point instructions, certain system calls), hopefully ruling out the
+;;; possibility that we would trigger it during allocation.
+
+(defun enable-interrupt (signal handler &key synchronous)