Update conform/Makefile mkdir commands.

conform/Makefile creates $(@D)/scratch for the per-standard per-header
tests.  That directory was formerly used by the Perl scripts for
temporary files, but the Python implementations use
tempfile.TemporaryDirectory to get such files cleaned up
automatically.  This patch changes the Makefile to create only $(@D)
(required for the output redirection to work), not the scratch
subdirectory.

Tested for x86_64.

	* conform/Makefile ($(conformtest-header-tests)): Create $(@D),
	not $(@D)/scratch.
	($(linknamespace-header-tests)): Likewise.