Move type_byte_order earlier

Move type_byte_order earlier

I failed to notice that the scalar_storage_order patch put
type_byte_order at the end of gdbtypes.c.  The end of the file is
normally where the file's _initialize function goes.  This moves
type_byte_order earlier, into a more relevant section.

gdb/ChangeLog
2019-12-04  Tom Tromey  <tromey@adacore.com>

	* gdbtypes.c (type_byte_order): Move earlier.  Assert for unknown
	endian-ness.

Change-Id: I4666431ecbb32ec98918f39f72d22c86b2bc8dde