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/cpu-features.c

@@ -0,0 +1,30 @@
+/* Initialize CPU feature data.  LoongArch64 version.
+   This file is part of the GNU C Library.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   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/>.  */
+
+#include <cpu-features.h>
+#include <elf/dl-hwcaps.h>
+#include <elf/dl-tunables.h>
+extern void TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *) attribute_hidden;
+
+static inline void
+init_cpu_features (struct cpu_features *cpu_features)
+{
+  GLRO(dl_larch_cpu_features).hwcap = GLRO(dl_hwcap);
+  TUNABLE_GET (glibc, cpu, hwcaps, tunable_val_t *,
+	       TUNABLE_CALLBACK (set_hwcaps));
+}