[PowerPC] Consolidate wordsize getter between native and gdbserver

[PowerPC] Consolidate wordsize getter between native and gdbserver

This patch moves the native target wordsize getter for ppc linux to
nat/ so that it can be used to simplify ppc_arch_setup in
gdbserver. The ptrace call used to get MSR for this is ultimately the
same as before, but it is no longer necessary to create a temporary
regcache to call fetch_inferior_registers.

gdb/ChangeLog:
2018-05-22  Pedro Franco de Carvalho  <pedromfc@linux.vnet.ibm.com>

	* configure.nat <linux powerpc>: Add ppc-linux.o to NATDEPFILES.
	* ppc-linux-nat.c (ppc_linux_target_wordsize): Move to
	nat/ppc-linux.c.
	(ppc_linux_nat_target::auxv_parse): Get thread id tid. Call
	ppc_linux_target_wordsize with tid.
	(ppc_linux_nat_target::read_description): Call ppc_linux_target
	wordsize with tid.
	* nat/ppc-linux.c: Include nat/gdb_ptrace.h.
	(ppc64_64bit_inferior_p): Add static and inline specifiers.
	(ppc_linux_target_wordsize): Move here from ppc-linux-nat.c. Add
	tid parameter. Remove static specifier.
	* nat/ppc-linux.h (ppc64_64bit_inferior_p): Remove declaration.
	(ppc_linux_target_wordsize): New declaration.

gdb/gdbserver/ChangeLog:
2018-05-22  Pedro Franco de Carvalho  <pedromfc@linux.vnet.ibm.com>

	* linux-ppc-low.c (ppc_arch_setup): Remove code for getting the
	wordsize of the inferior. Call ppc_linux_target_wordsize.