gdbarch: use predefault for more value components within gdbarch

For some reason the following value components of gdbarch:

  bfloat16_format
  half_format
  float_format
  double_format
  long_double_format
  so_ops

All use a postdefault but no predefault to set the default value for
the component.

As the postdefault values for these components are all constant
pointers that don't depend on other fields within the gdbarch, then I
don't see any reason why we couldn't use a predefault instead.

So lets do that.

Approved-By: Simon Marchi <simon.marchi@efficios.com>