Implement some missing functions in drivers/acpi.cc, which an OS that
uses the ACPICA library needs to implement, to enable the use of
semaphores and locks.

These functions get called from ACPICA functions for entering sleep
state - and in particular for powering off - which we will use in the
next patch.

This patch includes no new implementation - the semaphore implementation
was already committed earlier, and here it is just used.