x86: fix build with gcc 4.7

The upcoming gcc 4.7 has more advanced constant propagation
resulting some inline asm operands becoming constants and thus
emitted as literals, sometimes in contexts where this results
in invalid instructions.

This patch changes the constraints of the relevant operands
to "rm" thus forcing a valid type.  While obviously suboptimal,
this is what older gcc versions already did, and there is no
change to the code generated with these.

Signed-off-by: Mans Rullgard <mans@mansr.com>
