Add a DTRACEFLAGS variable, which can be used to pass additional variables
to dtrace(1) invocations during a build. This change includes -C in the
default flags, which has dtrace(1) run input scripts through the
preprocessor. While here, sort the definitions of CP and CPP in sys.mk.

Differential Revision: https://reviews.freebsd.org/D2204
Reviewed by: imp, rpaulo (previous revision)