libstdc++: Fix testsuite no_pch directive

The { dg-add-options no_pch } directive is supposed to add a macro
definition that invalidates the PCH file, and ensures that the #include
directives in the test file are processed as written. But the proc that
adds the options actually removes all existing options, cancelling out
any previous dg-options directive.

This means that using no_pch will cause FAILs in a file that relies on
other options set by an earlier dg-options.

The no_pch directive was added for PR libstdc++/21769 where Janis
suggested adding it as return "$flags -D__GLIBCXX__=99999999" but what
was actually committed didn't include the $flags so replaced them.

Additionally, using no_pch  only prevents the precompiled version of
<bits/stdc++.h> from being included, it doesn't prevent the
non-precompiled version being included by -include bits/stdc++.h in the
test flags. Use regsub to filter that out of the options as well.

libstdc++-v3/ChangeLog:

	* testsuite/lib/dg-options.exp (add_options_for_no_pch): Remove
	any "-include bits/stdc++.h" from options and add the macro to
	the existing options instead of replacing them.