tcg-llvm: fix LLVM implementation of TCG op_div2

The LLVM code implementing op_div2 and op_divu2 was broken:
the dividend was bits*2 wide but the divisor was only bits wide.
This fixes the problem by extending the divisor
and then truncating the quotient.

In addition, an incorrect use of #ifdef TCG_TARGET_HAS_div_i32
was fixed (TCG_TARGET_HAS_div_i32 is always defined but may be 0 or 1).
Ubuntu 14.04 compatibility

Had to disable libfdt in qemu, as there are some issues with the header files in 14.04.