conformtest: Enable tests when cross compiling.

This patch enables the compilation part of the conformtest tests (the
vast bulk of them) when cross compiling, so making it easy to run them
across many configurations with build-many-glibcs.py.

Tested with build-many-glibcs.py.

	* conform/Makefile (tests-special): Do not make addition of
	$(conformtest-header-tests) conditional on [$(cross-compiling) = no].
	(generated): Do not make addition of $(conformtest-header-base)
	conditional on [$(cross-compiling) = no].