Implement proper support for `-falign-functions`

This implements support for the previously ignored flag
`-falign-functions`.  This allows the frontend to request alignment on
function definitions in the translation unit where they are not
explicitly requested in code.  This is compatible with the GCC behaviour
and the ICC behaviour.

The scalar value passed to `-falign-functions` aligns functions to a
power-of-two boundary.  If flag is used, the functions are aligned to
16-byte boundaries.  If the scalar is specified, it must be an integer
less than or equal to 4096.  If the value is not a power-of-two, the
driver will round it up to the nearest power of two.

llvm-svn: 330378