mangle_style: Add new DWARF5 constants

Update bfd/dwarf2.c with the post DWARF5 language codes which
were added after DWARF5 was finalized. Adding them makes it
possible to return the mangling style for the new language
codes for Ada 2005 Fortran, C++, C and Assembly.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Jan Beulich <jbeulich@suse.com>