[compiler-rt][test] Make glibc-* feature detection work on a musl distribution

... where `ldd --version` has empty stdout and non-empty stderr.

compiler-rt/test/lit.common.cfg.py

@@ -363,7 +363,7 @@ if config.android:
   for required in [26, 28, 30]:
     if android_api_level >= required:
       config.available_features.add('android-%s' % required)
-  # FIXME: Replace with appropriate version when availible.  
+  # FIXME: Replace with appropriate version when availible.
   if android_api_level > 30 or (android_api_level == 30 and android_api_codename == 'S'):
     config.available_features.add('android-thread-properties-api')
 
@@ -380,18 +380,18 @@ else:
 
 if config.host_os == 'Linux':
   # detect whether we are using glibc, and which version
-  # NB: 'ldd' is just one of the tools commonly installed as part of glibc
+  # NB: 'ldd' is just one of the tools commonly installed as part of glibc/musl
   ldd_ver_cmd = subprocess.Popen(['ldd', '--version'],
                                  stdout=subprocess.PIPE,
                                  env={'LANG': 'C'})
   sout, _ = ldd_ver_cmd.communicate()
-  ver_line = sout.splitlines()[0]
-  if not config.android and ver_line.startswith(b"ldd "):
+  ver_lines = sout.splitlines()
+  if not config.android and len(ver_lines) and ver_lines[0].startswith(b"ldd "):
     from distutils.version import LooseVersion
-    ver = LooseVersion(ver_line.split()[-1].decode())
+    ver = LooseVersion(ver_lines[0].split()[-1].decode())
     for required in ["2.27", "2.30"]:
       if ver >= LooseVersion(required):
-        config.available_features.add("glibc-" + required)  
+        config.available_features.add("glibc-" + required)
 
 sancovcc_path = os.path.join(config.llvm_tools_dir, "sancov")
 if os.path.exists(sancovcc_path):