tests/tcg: better detect confused gdb which can't connect

While we may gamely give the right information it can still confuse
the wide range of GDBs out there. For example ppc64abi32-linux-user
reports:

  warning: Selected architecture powerpc:common is not compatible with reported target architecture powerpc:common64
  warning: Architecture rejected target-supplied description

but still connects. Add a test for a 0 pc and exit early if that is
the case. This may actually be a bug we need to fix?

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200520140541.30256-8-alex.bennee@linaro.org>

tests/tcg/multiarch/gdbstub/sha1.py

@@ -65,6 +65,10 @@ except (gdb.error, AttributeError):
     print("SKIPPING (not connected)", file=sys.stderr)
     exit(0)
 
+if gdb.parse_and_eval('$pc') == 0:
+    print("SKIP: PC not set")
+    exit(0)
+
 try:
     # These are not very useful in scripts
     gdb.execute("set pagination off")