sysent: regenerate files every time

This forces everything to be regenerated by marking makesyscalls.lua
as .PHONY, which may be helpful in, e.g., rebase scenarios, which may
have unexpected effects on mtimes but still really need a regen.

For now we apply this to the default sysent target, which is ran
manually and not as a part of buildkernel. We can reconsider this if
someone manually running it has issues with the runtime, but the top
level sysent target can run each in parallel; I recommend -j4 or so.

Requested by: kib
Reviewed by: brooks, kib (both earlier version)
Differential Revision: https://reviews.freebsd.org/D33357