Remove all remaining SPL code. Replace the mtd_cpl field in the machine

Remove all remaining SPL code.  Replace the mtd_cpl field in the machine
dependant thread structure and the CPL field in the interrupt stack frame
with dummies (so structural sizes do not change, yet).  Remove all interrupt
handler SPL mask and mask pointer code.  Remove all spl*() functions except
for splz().

Note that doreti uses a temporary CPL mask internally to accumulate a bitmap
of FAST interrupts which could not be executed due to not being able to get
the BGL.  This mask has no outside visibility.

Note that gd_fpending and gd_ipending still exist to support critical section
interrupt deferment.