[libcxx] Handle target triples with dashes in platform name

[miyuki]

Summary

Target triples may contain a dash in the platform name (e.g.
"aarch64-arm-none-eabi"). Account for it when splitting the triple
into components.

[ldionne]

I thought use_system_cxx_lib was only used on Apple platforms!

libcxx/utils/libcxx/test/config.py

def configure_features(self):
             for f in additional_features.split(','):
                 self.config.available_features.add(f.strip())
 
         # Write an "available feature" that combines the triple when
         # use_system_cxx_lib is enabled. This is so that we can easily write
         # XFAIL markers for tests that are known to fail with versions of
         # libc++ as were shipped with a particular triple.
         if self.use_system_cxx_lib:
-            (arch, vendor, platform) = self.config.target_triple.split('-')
+            (arch, vendor, platform) = self.config.target_triple.split('-', 2)
             (sysname, version) = re.match(r'([^0-9]+)([0-9\.]*)', platform).groups()
 
             self.config.available_features.add('with_system_cxx_lib={}-{}-{}{}'.format(arch, vendor, sysname, version))
             self.config.available_features.add('with_system_cxx_lib={}{}'.format(sysname, version))
             self.config.available_features.add('with_system_cxx_lib={}'.format(sysname))
 
             self.config.available_features.add('availability={}'.format(sysname))
             self.config.available_features.add('availability={}{}'.format(sysname, version))