gas emits incorrect .debug_line prologue header length for 64-bit DWARF

[Ed Maste]

The header_length field is supposed to be the number of bytes
following the field to the first byte of the line number program, but
dwarf2dbg.c:out_debug_line has hard-coded constants that are correct
only for 32-bit DWARF.

I've fixed it in FreeBSD here in SVN r256692:
http://svnweb.freebsd.org/base?view=revision&revision=256692

-Ed