C6X: remove dead code from entry.S

The ENDPROC() on sys_fadvise64_c6x() in arch/c6x/kernel/entry.S is
outside of the conditional block with the matching ENTRY() macro. This
leads a newer (v2.22 vs. v2.20) assembler to complain:

  /tmp/ccGZBaPT.s: Assembler messages:
  /tmp/ccGZBaPT.s: Error: .size expression for sys_fadvise64_c6x does not evaluate to a constant

The conditional block became dead code when c6x switched to generic
unistd.h and should be removed along with the offending ENDPROC().

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>