Pull out common parts of _initialize_{i386,amd64}_linux_nat

Pull out common parts of _initialize_{i386,amd64}_linux_nat

This commit adds two new helpers, x86_linux_create_target and
x86_linux_add_target, to hold the parts of _initialize_i386_linux_nat
and _initialize_amd64_linux_nat which are common.

gdb/
2014-07-11  Gary Benson  <gbenson@redhat.com>

	* amd64-linux-nat.c (x86_linux_create_target): New function.
	(x86_linux_add_target): Likewise.
	(_initialize_amd64_linux_nat): Delegate to the above new functions.
	* i386-linux-nat.c (x86_linux_create_target): New function.
	(x86_linux_add_target): Likewise.
	(_initialize_i386_linux_nat): Delegate to the above new functions.