x86: Modify ENTRY in sysdep.h so that p2align can be specified

No bug.

This change adds a new macro ENTRY_P2ALIGN which takes a second
argument, log2 of the desired function alignment.

The old ENTRY(name) macro is just ENTRY_P2ALIGN(name, 4) so this
doesn't affect any existing functionality.

Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>