[PowerPC] Move up some register access routines

Keep the routines related to register access grouped together.

gdb/ChangeLog:
2020-03-30  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* ppc-linux-nat.c (ppc_linux_nat_target::store_registers)
	(ppc_linux_nat_target::auxv_parse)
	(ppc_linux_nat_target::read_description)
	(supply_gregset, fill_gregset, supply_fpregset, fill_fpregset):
	Move up.