pretty/asm.rs test is failing on AArch64

[pietroalbini]

The src/test/pretty/asm.rs test is failing on AArch64.

2020-06-08T14:19:35.3826793Z error: invalid register `al`: unknown register
2020-06-08T14:19:35.3827908Z   --> /checkout/src/test/pretty/asm.rs:17:18
2020-06-08T14:19:35.3828376Z    |
2020-06-08T14:19:35.3828948Z 17 |         asm!("", out("al") _, lateout("rbx") _);
2020-06-08T14:19:35.3829292Z    |                  ^^^^^^^^^^^
2020-06-08T14:19:35.3829460Z 
2020-06-08T14:19:35.3829755Z error: invalid register `rbx`: unknown register
2020-06-08T14:19:35.3830643Z   --> /checkout/src/test/pretty/asm.rs:17:31
2020-06-08T14:19:35.3831010Z    |
2020-06-08T14:19:35.3831309Z 17 |         asm!("", out("al") _, lateout("rbx") _);
2020-06-08T14:19:35.3831632Z    |                               ^^^^^^^^^^^^^^^^
2020-06-08T14:19:35.3831812Z 
2020-06-08T14:19:35.3832106Z error: aborting due to 2 previous errors

We should avoid having x86_64-specific registers in that test.

This issue has been assigned to @yerke via this comment.

[pietroalbini]

cc @Stammark @JamieCunliffe @vigoux @joaopaulocarreiro @raw-bin, this is one of the tests that are failing on AArch64.

[nagisa]

cc @Amanieu

[Amanieu]

I think it's just a matter of marking the test as // only-x86_64

[yerke]

@rustbot claim