Rewrite the JDIMENSION argument management for x86 64-bit assembly to

Rewrite the JDIMENSION argument management for x86 64-bit assembly to
correctly handle a 32-bit argument width. The ABI leaves the high 32
bits undefined when passing 32-bit integers through arguments.

Also enhance the documentation of JDIMENSION to explain that its size
is significant to the implementation of the assembly.


This prevents libjpeg_turbo from crashing when compiled with recent
versions of Clang. This patch has been sent upstream:
https://github.com/libjpeg-turbo/libjpeg-turbo/pull/20

BUG=532214
R=thakis@chromium.org

Committed: https://chromium.googlesource.com/chromium/deps/libjpeg_turbo/+/52e8b6949a23a95fe1b6118805579e9aea268c3e

[hans, 2015-09-16 16:54:25]

hans@chromium.org changed reviewers:
+ thakis@chromium.org

[hans, 2015-09-16 16:54:26]

Please take a look.

I also learned that some of these files (e.g. jdclrss2-64.asm) are only included
in other files. The build system doesn't know about this dependency (and it's
not obvious how to track - yasm provides a -M flag but not -MD).

This should not be a problem for us in practice, because we need this fix for
rolling Clang, which clobbers the whole build. But it's still scary.

[Nico, 2015-09-16 17:20:21]

lgtm

[hans, 2015-09-16 17:24:00]

Committed patchset #2 (id:20001) manually as
52e8b6949a23a95fe1b6118805579e9aea268c3e (presubmit successful).