LoongArch: Add glibc.cpu.hwcap support.

The current IFUNC selection is always using the most recent
features which are available via AT_HWCAP.  But in
some scenarios it is useful to adjust this selection.

The environment variable:

GLIBC_TUNABLES=glibc.cpu.hwcaps=-xxx,yyy,zzz,....

can be used to enable HWCAP feature yyy, disable HWCAP feature xxx,
where the feature name is case-sensitive and has to match the ones
used in sysdeps/loongarch/cpu-tunables.c.

Signed-off-by: caiyinyu <caiyinyu@loongson.cn>

sysdeps/unix/sysv/linux/loongarch/libc-start.c

@@ -0,0 +1,34 @@
+/* Override csu/libc-start.c on LoongArch64.
+   Copyright (C) 2024 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
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SHARED
+
+/* Mark symbols hidden in static PIE for early self relocation to work.  */
+#if BUILD_PIE_DEFAULT
+# pragma GCC visibility push (hidden)
+#endif
+
+#include <ldsodefs.h>
+#include <cpu-features.c>
+
+extern struct cpu_features _dl_larch_cpu_features;
+
+#define ARCH_INIT_CPU_FEATURES() init_cpu_features (&_dl_larch_cpu_features)
+
+#endif
+#include <csu/libc-start.c>