MFC r340242:

Add a top-level make target to rebuild all sysent files.

The sysent target is useful when changing makesyscalls.sh, when
making paired changes to syscalls.master files, or in a future where
freebsd32 sysent entries are built from the default syscalls.master.

Reviewed by: bdrewery
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17899