[libc++] Check that gdb is linked with Python 3

[pirama]

Summary

The tests rely on python3 so fail when run against a gdb built with
Python 2.

[arichardson]

Do they still fail after rG89103bd2f06d? I was able to run ninja check-all successfully with a gdb linked against 2.7.

[pirama]

In D117728#3256550, @arichardson wrote:

Do they still fail after rG89103bd2f06d? I was able to run ninja check-all successfully with a gdb linked against 2.7.

This no longer fails with your fix. Thanks @arichardson.

libcxx/utils/libcxx/test/features.py

def check_gdb(cfg):
   gdb_path = shutil.which('gdb')
   if gdb_path is None:
     return False
 
   # Check that we can set breakpoint commands, which was added in 8.3.
   # Using the quit command here means that gdb itself exits, not just
   # the "python <...>" command.
   test_src = """\
+import sys
+if sys.version_info.major < 3:
+  sys.exit(1)
 try:
   gdb.Breakpoint(\"main\").commands=\"foo\"
 except AttributeError:
   gdb.execute(\"quit 1\")
 gdb.execute(\"quit\")"""
 
   try:
     stdout = subprocess.check_output(