Fix Ada value printing on PPC64

Fix Ada value printing on PPC64

The val_print removal patches introduced an Ada regression on PPC64
(probably any big-endian system).

The issue comes because value_field does not understand that Ada
wrapper fields can be bitfields that wrap a non-scalar type.  In this
case the value is already left-justified, so the justification done
there does the wrong thing.

Perhaps it would be good, eventually, to change value_field to
understand this case.  In the meantime this implements an Ada-specific
solution.

gdb/ChangeLog
2020-05-11  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_value_primitive_field): Now public.
	* ada-lang.h (ada_value_primitive_field): Declare.
	* ada-valprint.c (print_field_values): Use
	ada_value_primitive_field for wrapper fields.