Extract suffix rules into bsd.suffixes[-posix].mk.

Refactor make suffix rules into separate files (one for POSIX and one not),
and rationalise the rules so that bsd.lib.mk can contain only those rules
that are library-specific (.c.po and .c.pico).

This can be accomplished by adding ${STATIC_CFLAGS} to the .c.o rule
unconditionally. STATIC_CFLAGS are only defined for use by sys.mk rules in
lib/libpam/Makefile.inc (see r227797), so it should be safe to include
them unconditionally in sys.mk's .c.o rule (tested by make universe and a
ports exp-run).

Reviewed by: bdrewery, sjg
Approved by: rwatson (mentor)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D6805