Fix discarded-qualifiers problems in ldlang.c

Assign strchr return to a const char* to match its arg in a couple of
places.  archive_path now returns a const char*, and
input_statement_is_archive_path now has a const char* sep arg.  If
you follow where these args come from in ldgram.y it can be seen that
they are in fact in writable memory, so it isn't necessary to copy
file_spec to poke in a zero which is restored before the function
returns.

Signed-off-by: Calvin Owens <calvin@wbinvd.org>
Signed-off-by: Alan Modra <amodra@gmail.com>