src.conf: Introduce WITHOUT_MACHDEP knob.

Summary:
This knob can be used to make buildsystem prefer generic C implentations of
various functions, instead of machine-specific assembler ones.

Test Plan: `make buildworld` on amd64

Reviewed by: imp, emaste

Differential Revision: https://reviews.freebsd.org/D36076

MFC after: 3 days