String: Add support for __memcmpeq() ABI on all targets

No bug.

This commit adds support for __memcmpeq() as a new ABI for all
targets. In this commit __memcmpeq() is implemented only as an alias
to the corresponding targets memcmp() implementation. __memcmpeq() is
added as a new symbol starting with GLIBC_2.35 and defined in string.h
with comments explaining its behavior. Basic tests that it is callable
and works where added in string/tester.c

As discussed in the proposal "Add new ABI '__memcmpeq()' to libc"
__memcmpeq() is essentially a reserved namespace for bcmp(). The means
is shares the same specifications as memcmp() except the return value
for non-equal byte sequences is any non-zero value. This is less
strict than memcmp()'s return value specification and can be better
optimized when a boolean return is all that is needed.

__memcmpeq() is meant to only be called by compilers if they can prove
that the return value of a memcmp() call is only used for its boolean
value.

All tests in string/tester.c passed. As well build succeeds on
x86_64-linux-gnu target.

1 files changed, 3 insertions, 0 deletions

diff --git a/string/Versions b/string/Versions
index 298ecd401a..864c4cf7a4 100644
--- a/string/Versions
+++ b/string/Versions
@@ -89,4 +89,7 @@ libc {
     sigdescr_np; sigabbrev_np;
     strerrordesc_np; strerrorname_np;
   }
+  GLIBC_2.35 {
+    __memcmpeq;
+  }
 }