x86: Add support for building {w}memcmp{eq} with explicit ISA level

1. Refactor files so that all implementations are in the multiarch
   directory
    - Moved the implementation portion of memcmp sse2 from memcmp.S to
      multiarch/memcmp-sse2.S

    - The non-multiarch file now only includes one of the
      implementations in the multiarch directory based on the compiled
      ISA level (only used for non-multiarch builds.  Otherwise we go
      through the ifunc selector).

2. Add ISA level build guards to different implementations.
    - I.e memcmp-avx2-movsb.S which is ISA level 3 will only build if
      compiled ISA level <= 3. Otherwise there is no reason to include
      it as we will always use one of the ISA level 4
      implementations (memcmp-evex-movbe.S).

3. Add new multiarch/rtld-{w}memcmp{eq}.S that just include the
   non-multiarch {w}memcmp{eq}.S which will in turn select the best
   implementation based on the compiled ISA level.

4. Refactor the ifunc selector and ifunc implementation list to use
   the ISA level aware wrapper macros that allow functions below the
   compiled ISA level (with a guranteed replacement) to be skipped.

Tested with and without multiarch on x86_64 for ISA levels:
{generic, x86-64-v2, x86-64-v3, x86-64-v4}

And m32 with and without multiarch.

sysdeps/x86/cpu-tunables.c

@@ -30,7 +30,8 @@
    since IFUNC must be set up by init_cpu_features.  */
 # if defined USE_MULTIARCH && !defined SHARED
 #  ifdef __x86_64__
-#   define DEFAULT_MEMCMP	__memcmp_sse2
+/* DEFAULT_MEMCMP by sysdeps/x86_64/memcmp-isa-default-impl.h.  */
+#   include <sysdeps/x86_64/memcmp-isa-default-impl.h>
 #  else
 #   define DEFAULT_MEMCMP	__memcmp_ia32
 #  endif