src.conf: Introduce WITHOUT_MACHDEP_OPTIMIZATIONS 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

(cherry picked from commit 20adba8bc1beb125d5e5ed8f12e747ae79ca6a10)