mlx4, mthca: Silence warnings about no-op alignment operations

Since commit 8fa6abb6f4f64f ("Expose clang's alignment builtins and use
them for roundup2/rounddown2"), clang emits warnings for several
alignment operations in these drivers because the operation is a no-op.
The compiler is arguably being too strict here, but in the meantime
let's silence the warnings by conditionally compiling the alignment
operations.

Reviewed by: arichardson, hselasky
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28576