gallivm: fix 2 leaks in disassembly code

gallivm: fix 2 leaks in disassembly code

don't leak the MCSubtargetInfo (not really big, was already fixed with
llvm master) and TargetMachine (big). While this is only used for debugging
the leak is large enough to get you into trouble in some cases.
Tested with llvm 3.1 and master.
Before (llvm 3.1), GALLIVM_DEBUG=asm glxgears:
==14152== LEAK SUMMARY:
==14152==    definitely lost: 105,228 bytes in 20 blocks
==14152==    indirectly lost: 347,252 bytes in 261 blocks
==14152==      possibly lost: 866,625 bytes in 1,453 blocks
==14152==    still reachable: 7,344,677 bytes in 6,494 blocks
==14152==         suppressed: 0 bytes in 0 blocks

After:
==13799== LEAK SUMMARY:
==13799==    definitely lost: 3,108 bytes in 6 blocks
==13799==    indirectly lost: 0 bytes in 0 blocks
==13799==      possibly lost: 804,143 bytes in 1,429 blocks
==13799==    still reachable: 7,314,267 bytes in 6,473 blocks
==13799==         suppressed: 0 bytes in 0 blocks

Reviewed-by: Brian Paul <brianp@vmware.com>