Add (currently failing) test to check for correct long double alignment in varargs on x86_64.
Fixed passing of long double (and other big types) in var_args on x86_64. Removed XFAIL tag from the Feature/VarArgLongDouble.c test

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.

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.
Merge pull request #168 from willemp/fix-va-args-passing-for-big-types

Fix va args passing for big types