[COMMIT] A few imcc tweaks

[Melvin Smith]

As discussed in the last IMCC RFC, I've committed a few of the changes.

IMCC will now generate an error if the register type is unknown
rather than just assign it a PMC register.

P reg types are pmc, object, or a valid classname. Use pmc or
object for "generic" P register to defer type checking.

I think a few compilers will now be broken but the patches should
be very simple to fix them.

Also allow sub names without _ prepended, and allow @ to start labels.
Tweak the fixup code a bit to use flags rather than looking for _ in symbol
name.

Added parser stubs for .global to grammar.

-Melvin