LoongArch: Add ifunc support for memrchr{lsx, lasx}

According to glibc memrchr microbenchmark, this implementation could reduce
the runtime as following:

Name            Percent of rutime reduced
memrchr-lasx    20%-83%
memrchr-lsx     20%-64%

1 files changed, 40 insertions, 0 deletions

diff --git a/sysdeps/loongarch/lp64/multiarch/ifunc-memrchr.h b/sysdeps/loongarch/lp64/multiarch/ifunc-memrchr.h
new file mode 100644
index 0000000000..8215f9ad94
--- /dev/null
+++ b/sysdeps/loongarch/lp64/multiarch/ifunc-memrchr.h
@@ -0,0 +1,40 @@
+/* Common definition for memrchr implementation.
+   All versions must be listed in ifunc-impl-list.c.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <ldsodefs.h>
+#include <ifunc-init.h>
+
+#if !defined __loongarch_soft_float
+extern __typeof (REDIRECT_NAME) OPTIMIZE (lasx) attribute_hidden;
+extern __typeof (REDIRECT_NAME) OPTIMIZE (lsx) attribute_hidden;
+#endif
+extern __typeof (REDIRECT_NAME) OPTIMIZE (generic) attribute_hidden;
+
+static inline void *
+IFUNC_SELECTOR (void)
+{
+#if !defined __loongarch_soft_float
+  if (SUPPORT_LASX)
+    return OPTIMIZE (lasx);
+  else if (SUPPORT_LSX)
+    return OPTIMIZE (lsx);
+  else
+#endif
+    return OPTIMIZE (generic);
+}