Handle an edge case for minisym TLS variable lookups.

Handle an edge case for minisym TLS variable lookups.

If a TLS variable is provided by a minisym from a separate debug file,
the separate debug file is passed to
gdbarch_fetch_tls_load_module_address.  However, the object files
stored in the shared object list are the original object files, not
the separate debug object files.  In this case,
svr4_fetch_objfile_link_map was failing to find the link map entry
since the debug object file is not in its internal list, only the
original object file.

gdb/ChangeLog:

	* solib-svr4.c (svr4_fetch_objfile_link_map): Look for
	objfile->separate_debug_objfile_backlink if not NULL.