Use configure_lit_site_cfg instead of configure_file.

configure_lit_site_cfg defines some more parameters that are used in
lit.site.cfg.in. configure_file would leave those empty. These additional
definitions seem to be unimportant for regression tests, but unittests do not
work without them.

In case of out-of-tree builds, define the additional parameters with default
values. These may not take all configuration parameters into account, as
configure_lit_site_cfg would.

llvm-svn: 279733