conformtest: Skip execution tests when cross-compiling.

This patch makes conformtest skip execution tests when
cross-compiling, as an interim step towards running most of these
tests (presently disabled) in that case.  It omits the (obvious)
Makefile change to actually enable the conformtest tests when
cross-compiling, as there are still enough failures seen with
build-many-glibcs.py that I'd like to get the results cleaner before
enabling these tests.

Tested for x86_64, and with the tests actually enabled for
cross-compilation with build-many-glibcs.py.

	* conform/conformtest.pl ($cross): New variable.
	(--cross): New command-line option.
	(runtest): Skip test execution when cross-compiling.
	* conform/Makefile (conformtest-cross): New variable.
	($(conformtest-header-tests)): Pass $(conformtest-cross) to
	conformtest.pl.