gdbarch: remove some unneeded predefault="0" from gdbarch_components.py

I noticed that there are a bunch of 'predefault="0"' lines in
gdbarch_components.py, and that some (just some, not all) of these are
not needed.

The gdbarch is already zero initialized, but these lines seem to
exists so that we can know when to compare against "0" and when to
compare against "NULL".  At least, this seems to be useful in some
places in the generated code.

Specifically, if we remove the predefault="0" line from the
max_insn_length component then we end up generating a line like:

  gdb_assert (gdbarch->max_insn_length != NULL);

which doesn't compile as we compare a ULONGEST to NULL.

In this commit I remove all the predefault="0" lines that I claim are
obviously not needed.  These are lines for components that are not
Values (i.e. the component holds a function pointer anyway), or for
Value components that hold a pointer type, in which case using NULL is
fine.

The only changes after this commit are some fields that have nullptr
as their initial value, and gcore_bfd_target now compares to NULL not
0 in gdbarch_gcore_bfd_target_p, which, given the field is of type
'const char *', seems like an improvement.

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