Add data layout to target specification

The data layout had become optional at some point. Some time after that,
it started causing a compiler error, so I removed it. From the Rust
side, those changes are documented in the following issue:
rust-lang/rust#31367

This is the pull request that made the data layout non-optional again,
is this one:
rust-lang/rust#32939

I took the layout I added here from the Rust compiler code. The various
built-in ARM targets seem to have mostly[1] the same target layout, which
makes sense, as the target layout describes mostly hardware
characteristics that shouldn't change between operation systems.

The layout I copied is from the `arm-unknown-linux-gnueabi` target,
here:
https://github.com/rust-lang/rust/blob/253b7c1e1a919a6b722c29a04241d6f08ff8c79a/src/librustc_back/target/arm_unknown_linux_gnueabi.rs#L19

I double-checked with the LLVM documentation on data layouts, and
everything seems legit, as far as I can tell. I don't completely
understand everything about it, though, so I can't give a 100%
guarantee. The LLVM documentation on data layouts is located here:
http://llvm.org/docs/LangRef.html#data-layout

In any case, the program compiles and works fine with the new layout, so
I'm assuming it's correct.

[1] "Mostly", because the one exception I see is the name mangline
    option ("m:"), which I set to "e", meaning "ELF". This doesn't seem
    terribly relevant. The only case, that I can think of, that might
    make it relevant is if we had C code calling into our Rust code, but
    then we would mark the called Rust functions as "#[no_mangle]"
    anyway.

Author: hannobraun

blink/target.json

@@ -1,5 +1,6 @@
 {
     "llvm-target"         : "thumbv7m-unknown-none-eabi",
+    "data-layout"         : "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64",
     "linker"              : "arm-none-eabi-gcc",
     "pre-link-args"       : [ "-nostartfiles", "-Tlinker-script.ld" ],
     "post-link-args"      : [ "-l:libsam_sam3x8e_gcc_rel.a" ],