src.sys.obj.mk: Export OBJTOP

If we had to set OBJTOP we want to .export it (if .MAKE.LEVEL > 0)
for the benefit of things like x!= ${.MAKE} -r ... and similar.

Without OBJTOP, the default MAKEOBJDIR setting will result in
attempting to use an objdir like /lib or /bin etc in some cases.

This patch is based on D30990
with the addition of the check for level > 0.

Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D30990