ARM64: Work around Clang's inline assembly limitations.

ARM64: Work around Clang's inline assembly limitations.

Clang bug: https://llvm.org/bugs/show_bug.cgi?id=23575

This is the simplest workaround; just don't use the "J" constraint for
Clang. At worst, an extra `mov` will be generated to synthesize negative
values in a register.

BUG=

[jbramley, 2015-05-26 16:33:12]

jacob.bramley@arm.com changed reviewers:
+ bmeurer@chromium.org, ulan@chromium.org

[jbramley, 2015-05-26 16:33:12]

I realise that you may prefer to wait for a Clang fix for this issue. In that
case, it might be better to wait, since without this patch a fixed Clang could
generate marginally better code.

What do you think?

[Benedikt Meurer, 2015-05-26 17:42:15]

I'd prefer a clang fix. But in general the atomicops should be in sync with
Chromium base. Did you submit a CL for Chromium base as well?

[jbramley, 2015-05-28 06:18:10]

On 2015/05/26 17:42:15, Benedikt Meurer wrote:
> I'd prefer a clang fix. But in general the atomicops should be in sync with
> Chromium base. Did you submit a CL for Chromium base as well?

As discussed, let's wait for an updated Clang.