amd64: simplify addition of new general registers.

The purpose of this patch is only simplify the addition of new registers.
ORIG_RAX is kept as last register and any addition is done right before it.

2017-01-27  Walfred Tedeschi  <walfred.tedeschi@intel.com>

	* amd64-linux-tdep.h (AMD64_LINUX_ORIG_RAX_REGNUM):
	Set to AMD64_NUM_REGS.