mesa/macros: add power-of-two assertions for alignment macros

mesa/macros: add power-of-two assertions for alignment macros

ALIGN and ROUND_DOWN_TO both require that the alignment value passed
into the macro be a power of two in the comments. Using software assertions
verifies this to be the case.

v2: use static inline functions instead of gcc-specific statement expressions (Brian).
v3: fix indendation (Brian).
v4: add greater than zero requirement (Anuj).

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>