Remove use of vpath for implicit rules

File eh.c and tk.c are not in directory src/backend and can not be built
with pattern rule for BACK_OBJS. Adding explicit rule for these two
files solves the problem.

File strtold.c is in src/backend but strtold.o is assigned to DMD_OBJS
which pattern rule does not search src/backend.  Moving strtold.o to
BACK_OBJS solves the problem.