sys/sys: Fix __builtin_is_aligned fallback

When the compiler doesn't provide __builtin_is_aligned we use
macro as a fallback. The macro was missing brackets around one
argument. This could lead to incorrect results when the argument is
more complex than a single stagement.

Fix this by adding the needed brackets.

Reviewed by: brooks, imp, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45260