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Notes on porting to the alpha.

The main difficulty will be dealing with imprecise exceptions.  That can be
addressed mostly by just not generating exceptions.  Currently, we generate
exceptions in the following cases:

 - guard page for allocation overflow
 - fixnum+fixnum overflow
 - internal errors
 - pseudo-atomic-interrupted
 - %primitive halt
 - fp exceptions

Related, but not identical, is cleanly handling user interrupts (i.e.
signals).  We could poll like we plan to on the x86, but I would prefer not
taking the performance hit.

All the exceptions except for fp exceptions can easily be handled by just
explicitly checking for the condition and branching to a save-escape-frame
assembly routine if necessary.  Fp exceptions can be handled by adding trap
barrier instructions.  Exactly where and how often depends on the amount of
debug info desired.  For minimal debug info, at the end of every basic
block.  For maximal debug info, just after every fp op.  For medium, just
before every fp op.

User interrupts are tricky.