libada: Fix shared library installation with `--disable-libada'

Provide a default value of $(toolexeclibdir) for $(ADA_RTL_DSO_DIR), so
that in a `--disable-libada' configuration `make install' places shared
gnatlib libraries, built with `make -C gcc gnatlib-shared', in their
intended version-specific location, fixing a commit r276424 ("libada:
Respect `--enable-version-specific-runtime-libs'") regression.

	gcc/ada/
	* gcc-interface/Makefile.in (toolexeclibdir): New variable.

From-SVN: r279702