xen: arm: Add new str* primitives from Linux v3.16-rc6.

xen: arm: Add new str* primitives from Linux v3.16-rc6.

Imports:
  0a42cb0 arm64: lib: Implement optimized string length routines
    Author: zhichang.yuan <zhichang.yuan@linaro.org>
    Signed-off-by: Zhichang Yuan <zhichang.yuan@linaro.org>
    Signed-off-by: Deepak Saxena <dsaxena@linaro.org>
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
  192c4d9 arm64: lib: Implement optimized string compare routines
    Author: zhichang.yuan <zhichang.yuan@linaro.org>
    Signed-off-by: Zhichang Yuan <zhichang.yuan@linaro.org>
    Signed-off-by: Deepak Saxena <dsaxena@linaro.org>
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>