Makefile.tpl: Avoid race condition in generating site.exp from the top level

A command like "make -j 2 check-gcc-c check-gcc-c++" run in the top level of
a fresh build directory does not work reliably. That will spawn two
independent make processes inside the "gcc" directory, and each of those
will attempt to create site.exp if it doesn't exist and will interfere with
each other, producing often a corrupted or empty site.exp. Resolve that by
making these targets depend on a new phony target which makes sure site.exp
is created first before starting the recursive makes.

ChangeLog:

	* Makefile.in: Regenerate.
	* Makefile.tpl: Add dependency on site.exp to check-gcc-* targets