Exception handling should be enabled at all times during execution to
ensure that exceptions are not ignored which would cause further
problems. This separates use of the exception enable bit from use of the
interrupt enable bit in the machine status register so that they can be
manipulated independently.
This patch adds a vector for debug events along with a hook similar to
the exception framework. The debug vector generates an exception frame
for use by libdebugger.
This patch updates the CPU_Exception_frame to include all necessary
registers, combines hardware snd software exception handlers into a
shared vector, provides an architecture-specific hook for taking
control of exception handling, and moves exception handling over to
actually using the CPU_Exception_frame instead of a minimal interrupt
stack frame. As the significant contents of _exception_handler.S have
been entirely rewritten, the copyright information on this file has been
updated to reflect that.
