Change `(Registers[REGBLOCK_STACK_GUARD])' to be a cache for the C

Change `(Registers[REGBLOCK_STACK_GUARD])' to be a cache for the C
variable `Stack_Guard'.  Change COMPILER_SETUP_INTERRUPT so that
compiled code doesn't signal GC or stack-overflow interrupts when the
corresponding interrupt enable bit is not set.  Change compiled-code
interface to assume this, and eliminate back-out code; this eliminates
the _SKIPPED_CHECK_OFFSET constants built into the microcode, allowing
the compiled code more coding flexibility.