New codegen: GC calling convention must use registers.

New codegen: GC calling convention must use registers.

Previously, on register-deficient architectures like x86-32,
the new code generator would emit code for calls to stg_gc_l1,
stg_gc_d1 and stg_gc_f1 that pushed their single argument on
to the stack, while the functions themselves expected the
argument to live in L1, D1 and F1 (respectively).  This was
because cmmCall with the GC calling convention allocated real
registers, not virtual registers.

This patch modifies the code for assigning registers/stack slots
to use the right calling convention for GC and adds an assertion
to ensure it did it properly.