arm32/Makefile.inc: don't need to set .PATH
i386/Makefile.inc: don't need to set .PATH, SHLIB_VERSION_FILE
Fixes PR lib/8298 from Richard Earnshaw in a more general way.
Makefile.inc, always add the man pages for that directory, and only
specify a SRCS= line (at whatever else is needed for that arch) inside
a ${MACHINE_ARCH} .ifdef. The top-level Makefile includes <bsd.lib.mk>
only if SRCS is defined, and otherwise just includes <bsd.man.mk>.