arm: Implement memchr ifunc selection in C

arm: Implement memchr ifunc selection in C

This patch refactor ARM memchr ifunc selector to a C implementation.
No functional change is expected, including ifunc resolution rules.

It also reorganize the ifunc options code:

  1. The memchr_impl.S is renamed to memchr_neon.S and multiple
     compilation options (which route to armv6t2/memchr one) is
     removed.  The code to build if __ARM_NEON__ is defined is
     also simplified.

  2. A memchr_noneon is added (which as build along previous ifunc
     resolution) and includes the armv6t2 direct.

  3. Same as 2. for loader object.

Alongside the aforementioned changes, it also some cleanus:

  - Internal memchr definition (__GI_memcpy) is now a hidden
    symbol.
  - No need to create hidden definition for the ifunc variants.

Checked on armv7-linux-gnueabihf and with a build for arm-linux-gnueabi,
arm-linux-gnueabihf with and without multiarch support and with both
GCC 7.1 and GCC mainline.

	* sysdeps/arm/armv7/multiarch/Makefile [$(subdir) = string]
	(sysdeps_routines): Add memchr_noneon.
	* sysdeps/arm/armv7/multiarch/ifunc-memchr.h: New file.
	* sysdeps/arm/armv7/multiarch/memchr_noneon.S: Likewise.
	* sysdeps/arm/armv7/multiarch/rtld-memchr.S: Likewise.
	* sysdeps/arm/armv7/multiarch/memchr.S: Remove file.
	* sysdeps/arm/armv7/multiarch/memchr.c: New file.
	* sysdeps/arm/armv7/multiarch/memchr_impl.S: Move to ...
	* sysdeps/arm/armv7/multiarch/memchr_neon.S: ... here.

Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>