Write AArch64 big endian data fixup entries as BE.

There was support for writing the AArch64 big endian data fixup entries in
the .eh_frame section in BE.    This is changed to write all such fixup
entries in BE with no restriction on the section.  This is similar to
the existing support for fixup entries for ARM.

A test is added to check the length field in the .debug_line section as
this is an example of where such a fixup occurs.

Differential Revision: http://reviews.llvm.org/D16064

llvm-svn: 258320