Fix Primitive_GC to request the right amount of space from the GC.

Fix Primitive_GC to request the right amount of space from the GC.

New variable Free_primitive is set during primitive invocations to
the value of Free on entry, so that Primitive_GC can find the total
amount of space requested so far from the GC, rather than the amount
currently being requested.  Avoids endless GC loops, particularly in,
e.g., bignum primitives.