Fix gcrt0.o compilation

When static-start-installed-name is different from
start-installed-name, we must not use the shared objects.

* csu/Makefile
(extra-objs): Add gmon-start.o when building shared library and
$(static-start-installed-name) is different from
$(start-installed-name).
$(objpfx)g$(static-start-installed-name): When building shared
library and $(static-start-installed-name) is different from
$(static-start-installed-name), revert to non-shared rule,
i.e. using $(objpfx)% and gmon-start.o.