The `dofretu...*` intrinsics used %ac1 without declaring it as a used
registers, which effectively broke register allocation for handling
string/bytevector foreign-call results or callable arguments.
original commit: 993fb9036acad5445319f458fd971b1a1d8e9f84
Also, for completeness, correct the listing of callee-save registers
in callable return for x86 & x86_64.
original commit: 4cd942be6ab2eb5e02f6d6c5c509db3131bd015f
