Refactor endian handling in DFP routines

Refactor endian handling in DFP routines

This patch moves endian conversion into the decimal_from_number and
decimal_to_number routines, and removes it from all their callers,
making the code simpler overall.  No functional change.

gdb/ChangeLog:
2017-11-16  Ulrich Weigand  <uweigand@de.ibm.com>

	* target-float.c (decimal_from_number): Add byte_order argument and
	call match_endianness.  Error if unknown floating-point type.
	(decimal_to_number): Add byte_order argument and call match_endianness.
	(decimal_from_longest): Update call.  Do not call match_endianness.
	(decimal_from_ulongest): Likewise.
	(decimal_binop): Likewise.
	(decimal_is_zero): Likewise.
	(decimal_compare): Likewise.
	(decimal_convert): Likewise.