sysent: Reduce duplication and improve readability.

[brooks]

Summary

Use the power of variable to avoid spelling out source and generated
files multiple times. The previous Makefiles were hard to read, hard to
edit, and badly formatted.

[kevans]

sys/amd64/linux/Makefile

24–25

This one should also have a dependency on ${MAKESYSCALLS}

[brooks]

• Add missing dependency

[brooks]

• Improve whitespace consistency.