Restore linux_api's GETTID for ARM targets

This is a follow-up commit to 643b912
which fixed SeattleTestbed#98 (GETTID on MIPS) but broke it
for ARMs at the same time. GETTID's syscall number on ARM is identical
with x86_32's, or so my testing on a Raspberry Pi Model B Rev 2 with
BCM2835 SoC with an ARM CPU core indicates.

This commit also adds raising an exception to signal that we have no
GETTID number in case we are running on a platform other than
x86_32/64, MIPS, or ARM.

linux_api.py

@@ -200,7 +200,11 @@ def _get_machine_GETTID_number():
       GETTID = 224
   elif machine.startswith("mips"):
     GETTID = 4222
-  # TODO Needs ARM and possibly other clauses!
+  elif machine.startswith("arm"):
+    GETTID = 224
+  else:
+    raise Exception("I don't know the GETTID syscall number for machine '" +
+        machine + "' with architecture '" + architecture + "'.")
   return GETTID
 
 GETTID = _get_machine_GETTID_number()