Can't compile for thumbv6-none-eabi

[japaric]

Because some of our assembly implementation use thumb2 instructions that aren't available for it.

$ xargo build --target thumbv6m-none-eabi
   Compiling rustc_builtins v0.1.0 (file:///home/japaric/tmp/rustc-builtins)
error: <inline asm>:5:11: error: instruction requires: thumb2
          ldr r1, [sp], #4
          ^

  --> src/arm.rs:8:5
   |
8  |     asm!("push {lr}
   |     ^

error: <inline asm>:2:11: error: instruction requires: thumb2
          sub r12, sp, #12
          ^

  --> src/arm.rs:20:5
   |
20 |     asm!("push {lr}
   |     ^

error: <inline asm>:3:11: error: instruction requires: arm-mode
          str r12, [sp, #-20]!
          ^

  --> src/arm.rs:20:5
   |
20 |     asm!("push {lr}
   |     ^

error: <inline asm>:5:11: error: instruction requires: arm-mode
          ldrd r2, r3, [sp, #8]
          ^

  --> src/arm.rs:20:5
   |
20 |     asm!("push {lr}
   |     ^

error: aborting due to 4 previous errors

error: Could not compile `rustc_builtins`.

To learn more, run the command again with --verbose.
error: `cargo` process didn't exit successfully

Possible solutions:

• Change the assembly implementations to not use thumb2 instructions.
• Add a special implementation just for this target. The problem is that, AFAIK, there's no cfg argument that handles this conditional compilation. We'll likely need some way to cfg on the arch component of llvm-target.

cc @Amanieu