[Lightning] Subtract with borrow

[Mike Spivey]

The i386 implementation in GIT contains the definition for
subtract-with-carry:

#define jit_subxr_i(d, s1, s2)  \
    jit_opr_((d), (s1), (s2), SBBLrr((s1), (d)), SBBLrr((s2), (d)) )

Please forgive my not compiling a test, but doesn't this treat
subtraction as if it is commutative?  Correct code for the case where
s1 != d but s2 = d will be embarrassingly bad, but (if I'm right about
the error) bad code is better than wrong code, always!

Best wishes,

-- Mike