Expand the gdbarch per-architecture data vector as needed, rather

Expand the gdbarch per-architecture data vector as needed, rather
than requiring that all per-architecture data be registered before
the first gdbarch object is allocated.
* gdbarch.sh: Changes to effect the following:
* gdbarch.c (alloc_gdbarch_data, init_gdbarch_data): Delete
declarations and definitions.
(check_gdbarch_data): New function, and declaration.
(gdbarch_alloc): Don't call alloc_gdbarch_data; leaving the fields
zero is good enough.
(free_gdbarch_data): Tolerate a null data pointer.  Free only
those data items gdbarch->data actually has allocated.
(set_gdbarch_data, gdbarch_data): Call check_gdbarch_data.
(gdbarch_update_p): No need to call init_gdbarch_data.