gdb/gdbarch: split postdefault setup from invalid check in gdbarch.py

Restructure how gdbarch.py generates the verify_gdbarch function.
Previously the postdefault handling was bundled together with the
validation.  This means that a field can't have both a postdefault,
and set its invalid attribute to a string.

This doesn't seem reasonable to me, I see no reason why a field can't
have both a postdefault (used when the tdep doesn't set the field),
and an invalid expression, which can be used to validate the value
that a tdep might set.

In this commit I restructure the verify_gdbarch generation code to
allow the above, there is no change in the actual generated code in
this commit, that will come in later commit.

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