gdb: change linux gdbarch data from post to pre-init

gdb: change linux gdbarch data from post to pre-init

The following patch will need to fill a field in linux_gdbarch_data
while the gdbarch is being built.  linux_gdbarch_data is currently
allocated as a post-init gdbarch data, meaning it's not possible to fill
it before the gdbarch is completely initialized.  Change it to a
pre-init gdbarch data to allow this.

The init_linux_gdbarch_data function doesn't use the created gdbarch,
it only allocates the linux_gdbarch_data structure on the gdbarch's
obstack, so the change is trivial.

gdb/ChangeLog:

	* linux-tdep.c (init_linux_gdbarch_data): Change parameter to
	obkstack.
	(_initialize_linux_tdep): Register pre-init gdb data instead of
	post-init.

Change-Id: If35ce91b6bb5435680d43b9268d811d95661644f