Fix va args passing for big types

Fixed Executor to (more) correctly handle the alignment of types larger than 64bit (such as long double) when those are passed in var_args on x86_64.
Added test/Feature/VarArgLongDouble.c to test for the problem.
    
Specifically:
From http://www.x86-64.org/documentation/abi.pdf
AMD64-ABI 3.5.7p5: Step 7.
Align l->overflow_arg_area upwards to a 16 byte boundary if alignment needed by type exceeds 8 byte boundary.