x86: Add support for compiling {raw|w}memchr with high ISA level

1. Refactor files so that all implementations for in the multiarch
   directory.
    - Essentially moved sse2 {raw|w}memchr.S implementation to
      multiarch/{raw|w}memchr-sse2.S

    - The non-multiarch {raw|w}memchr.S 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 memchr-avx2.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 (memchr-evex{-rtm}.S).

3. Add new multiarch/rtld-{raw}memchr.S that just include the
   non-multiarch {raw}memchr.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.
    - Guranteed replacement essentially means that for any ISA level
      build there must be a function that the baseline of the ISA
      supports. So for {raw|w}memchr.S since there is not ISA level 2
      function, the ISA level 2 build still includes the ISA level
      1 (sse2) function. Once we reach the ISA level 3 build, however,
      {raw|w}memchr-avx2{-rtm}.S will always be sufficient so the ISA
      level 1 implementation ({raw|w}memchr-sse2.S) will not be built.

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.

1 files changed, 10 insertions, 0 deletions

diff --git a/sysdeps/x86_64/isa-default-impl.h b/sysdeps/x86_64/isa-default-impl.h
index 34634668e5..7d7832b1f5 100644
--- a/sysdeps/x86_64/isa-default-impl.h
+++ b/sysdeps/x86_64/isa-default-impl.h
@@ -46,4 +46,14 @@
 # error "Unsupported ISA Level!"
 #endif
 
+#if IS_IN(rtld)
+# if !defined USE_MULTIARCH
+#  error "RTLD version should only exist in multiarch build"
+# endif
+#else
+# if defined USE_MULTIARCH
+#  error "Multiarch build should not use ISA_DEFAULT_IMPL without RTLD"
+# endif
+#endif
+
 #include ISA_DEFAULT_IMPL