aarch64: Added Vector Length Set test helper script

This patch is a test helper script to change Vector Length for child
process. This script can be used as test-wrapper for 'make check'.

Usage examples:

~/build$ make check subdirs=string \
test-wrapper='~/glibc/sysdeps/unix/sysv/linux/aarch64/vltest.py 16'

~/build$ ~/glibc/sysdeps/unix/sysv/linux/aarch64/vltest.py 16 \
make test t=string/test-memcpy

~/build$ ~/glibc/sysdeps/unix/sysv/linux/aarch64/vltest.py 32 \
./debugglibc.sh string/test-memmove

~/build$ ~/glibc/sysdeps/unix/sysv/linux/aarch64/vltest.py 64 \
./testrun.sh string/test-memset

INSTALL

@@ -380,6 +380,10 @@ the same syntax as 'test-wrapper-env', the only difference in its
 semantics being starting with an empty set of environment variables
 rather than the ambient set.
 
+   For AArch64 with SVE, when testing the GNU C Library, 'test-wrapper'
+may be set to "SRCDIR/sysdeps/unix/sysv/linux/aarch64/vltest.py
+VECTOR-LENGTH" to change Vector Length.
+
 Installing the C Library
 ========================
 

manual/install.texi

@@ -418,6 +418,9 @@ use has the same syntax as @samp{test-wrapper-env}, the only
 difference in its semantics being starting with an empty set of
 environment variables rather than the ambient set.
 
+For AArch64 with SVE, when testing @theglibc{}, @samp{test-wrapper}
+may be set to "@var{srcdir}/sysdeps/unix/sysv/linux/aarch64/vltest.py
+@var{vector-length}" to change Vector Length.
 
 @node Running make install
 @appendixsec Installing the C Library

sysdeps/unix/sysv/linux/aarch64/vltest.py

@@ -0,0 +1,82 @@
+#!/usr/bin/python3
+# Set Scalable Vector Length test helper
+# Copyright (C) 2021 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/>.
+"""Set Scalable Vector Length test helper.
+
+Set Scalable Vector Length for child process.
+
+examples:
+
+~/build$ make check subdirs=string \
+test-wrapper='~/glibc/sysdeps/unix/sysv/linux/aarch64/vltest.py 16'
+
+~/build$ ~/glibc/sysdeps/unix/sysv/linux/aarch64/vltest.py 16 \
+make test t=string/test-memcpy
+
+~/build$ ~/glibc/sysdeps/unix/sysv/linux/aarch64/vltest.py 32 \
+./debugglibc.sh string/test-memmove
+
+~/build$ ~/glibc/sysdeps/unix/sysv/linux/aarch64/vltest.py 64 \
+./testrun.sh string/test-memset
+"""
+import argparse
+from ctypes import cdll, CDLL
+import os
+import sys
+
+EXIT_SUCCESS = 0
+EXIT_FAILURE = 1
+EXIT_UNSUPPORTED = 77
+
+AT_HWCAP = 16
+HWCAP_SVE = (1 << 22)
+
+PR_SVE_GET_VL = 51
+PR_SVE_SET_VL = 50
+PR_SVE_SET_VL_ONEXEC = (1 << 18)
+PR_SVE_VL_INHERIT = (1 << 17)
+PR_SVE_VL_LEN_MASK = 0xffff
+
+def main(args):
+    libc = CDLL("libc.so.6")
+    if not libc.getauxval(AT_HWCAP) & HWCAP_SVE:
+        print("CPU doesn't support SVE")
+        sys.exit(EXIT_UNSUPPORTED)
+
+    libc.prctl(PR_SVE_SET_VL,
+               args.vl[0] | PR_SVE_SET_VL_ONEXEC | PR_SVE_VL_INHERIT)
+    os.execvp(args.args[0], args.args)
+    print("exec system call failure")
+    sys.exit(EXIT_FAILURE)
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser(description=
+            "Set Scalable Vector Length test helper",
+            formatter_class=argparse.ArgumentDefaultsHelpFormatter)
+
+    # positional argument
+    parser.add_argument("vl", nargs=1, type=int,
+                        choices=range(16, 257, 16),
+                        help=('vector length '\
+                              'which is multiples of 16 from 16 to 256'))
+    # remainDer arguments
+    parser.add_argument('args', nargs=argparse.REMAINDER,
+                        help=('args '\
+                              'which is passed to child process'))
+    args = parser.parse_args()
+    main(args)