debug: accept GDB's main thread identifier (#657)

Recent GDB versions print 'Thread <main>' for remote targets that
expose a single synthetic thread. The thread parser only accepts
'Remote target' or numbered threads with names, so test setup
fails before the test body runs.

Signed-off-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com>

Author: jerryzj

debug/testlib.py

@@ -1067,7 +1067,8 @@ def threads(self):
             m = re.match(
                     r"[\s\*]*(\d+)\s*"
                     r'(Remote target'
-                    r'|Thread (\d+)\s*(?:".*?")?\s*\(Name: ([^\)]+))'
+                    r'|Thread (?:(\d+)|<main>)'
+                    r'(?:\s*(?:".*?")?\s*\(Name: ([^)]+)\))?)'
                     r"\s*(.*)", line)
             if m:
                 threads.append(Thread(*m.groups()))