Bug 1288313 - Explicitly reject unknown CPU types. r=chmanchester

Since bug 1264482, unknown CPU types end up triggering a ValueError
exception because of the CPU EnumString. Even if somehow the CPU is
valid, the endianness is not and would trigger a ValueError exception
as well.

So, instead of letting the exceptions happen, use a nicer failure mode
with an explicit die().

--HG--
extra : rebase_source : 68432496712075c677de4bf71ea5d420fc70c35c

build/moz.configure/init.configure

@@ -413,8 +413,7 @@ def split_triplet(triplet):
         canonical_cpu = 'aarch64'
         endianness = 'little'
     else:
-        canonical_cpu = cpu
-        endianness = 'unknown'
+        die('Unknown CPU type: %s' % cpu)
 
     return namespace(
         alias=triplet,