Fix build errors in aix-thread.c

This patch fixes a few instances of unused variable and shadowed local
variable errors.

gdb/ChangeLog:

	* aix-thread.c (ptid_cmp): Remove unused variable.
	(get_signaled_thread): Likewise.
	(store_regs_user_thread): Likewise.
	(store_regs_kernel_thread): Likewise.
	(fetch_regs_kernel_thread): Remove shadowed variable.