libc: Fix a few bugs in the xlocale collation code.
- Fix checks for mmap() failures. 
- Set the "map" and "maplen" fields of struct xlocale_collate so that the table destructor actually does something.
- Free an already-mapped collation file before loading a new one into the global table.
- Harmonize the prototype and definition of __collate_load_tables_l() by adding the "static" qualifier to the latter.