BFD: Fix double BFD_FAIL calls in `bfd_default_reloc_type_lookup'

BFD: Fix double BFD_FAIL calls in `bfd_default_reloc_type_lookup'

Break out of the outer switch statement once the inner switch for the
BFD_RELOC_CTOR relocation has been processed, preventing double BFD_FAIL
calls from being made, once from the inner switch and then again from
the default case of the outer switch.

Noticed with a `-Wimplicit-fallthrough' build error reported by a recent
GCC version:

In file included from .../bfd/reloc.c:52:0:
.../bfd/reloc.c: In function 'bfd_default_reloc_type_lookup':
.../bfd/libbfd.h:779:8: error: this statement may fall through [-Werror=implicit-fallthrough=]
   do { bfd_assert(__FILE__,__LINE__); } while (0)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../bfd/reloc.c:7780:4: note: in expansion of macro 'BFD_FAIL'
    BFD_FAIL ();
    ^~~~~~~~
.../bfd/reloc.c:7782:5: note: here
     default:
     ^~~~~~~
cc1: all warnings being treated as errors
make[4]: *** [reloc.lo] Error 1

	bfd/
	* reloc.c (bfd_default_reloc_type_lookup) <BFD_RELOC_CTOR>: Do
	not fall through to the default case.