metag: move usercopy.c exports out of metag_ksyms.c

It's less error prone to have function symbols exported immediately
after the function rather than in metag_ksyms.c. Move each EXPORT_SYMBOL
in metag_ksyms.c for symbols defined in usercopy.c into usercopy.c

Signed-off-by: James Hogan <james.hogan@imgtec.com>