MIPS64: Fix LoadGlobalViaContextStub

MIPS64: Fix LoadGlobalViaContextStub

Also make a minor improvement in the mips32 StoreGlobalViaContextStub
and fix some bad comments in the macro-assembler.

BUG=

Committed: https://crrev.com/35d6012b9f9f4afb6454d4d510ab7b1471a6907b
Cr-Commit-Position: refs/heads/master@{#29859}

[akos.palfi.imgtec, 2015-07-25 18:54:26]

akos.palfi@imgtec.com changed reviewers:
+ akos.palfi@imgtec.com

[akos.palfi.imgtec, 2015-07-25 18:54:27]

Lgtm

[paul.l..., 2015-07-26 04:38:57]

paul.lind@imgtec.com changed reviewers:
+ balazs.kilvady@imgtec.com, bmeurer@chromium.org, ishell@chromium.org

[paul.l..., 2015-07-26 04:39:02]

The CQ bit was checked by paul.lind@imgtec.com

[commit-bot: I haz the power, 2015-07-26 04:39:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1254153002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1254153002/1

[commit-bot: I haz the power, 2015-07-26 05:01:48]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-26 05:02:03]

Patchset 1 (id:??) landed as
https://crrev.com/35d6012b9f9f4afb6454d4d510ab7b1471a6907b
Cr-Commit-Position: refs/heads/master@{#29859}