demmt/macro: crude infinite loop detection

Blob sometimes uses macros which exit only when gpu object state
changes. For example:

0x03400115   read $r1 0xd00 [NVC1_3D.GRAPH.SCRATCH[0]]
0x00004211   mov $r2 0x1
0x00048910   mov $r1 (sub $r1 $r2)
0x00010827   braz annul $r1 4
0x00100071   maddrsend 0x40 [NVC1_3D.GRAPH.NOP]
0xffff4007   bra -3 (delayed)
0x03400115   read $r1 0xd00 [NVC1_3D.GRAPH.SCRATCH[0]]

It generates NOPs as long as SCRATCH[0] != 1.

Detect it by counting number of backward jumps and aborting simulation
when it reaches some arbitrary value (100 for now).