Open-code with-interrupt-mask, with-interrupts-reduced.

Open-code with-interrupt-mask, with-interrupts-reduced.

Not open-coded at the RTL level, but at the LAP level.

This way we avoid going through a return trampoline, which wrecks the
return address stack branch target predictor as long as we transition
between Scheme and C to handle trampolines.

Most of the work, of munging MEMTOP and STACK_GUARD, is relegated to
an assembly hook subroutine so the code doesn't expand too much.  The
format of the stack still uses reflect-to-interface so that this
should require no changes to the continuation parser to get the
interrupt masks right, but with an intermediate empty-frame
continuation that actually calls the assembly hook and then pops
reflect-to-interface off.