Add thumbv7em-linux-eabi target spec

Vladimir Pouzanov <notifications@github.com> writes:

>> @@ -0,0 +1,9 @@
>> +{
>> +    "data-layout": "e-m:e-p:32:32-i1:8:32-i8:8:32-i16:16:32-i64:64-v128:64:128-a:0:32-n32-S64",
>> +    "llvm-target": "thumbv7em-linux-eabi",
>
> Shouldn't this be `thumbv7em-none-eabi`?
>
Perhaps but I just used the target that rake calls for.