A64: Add and use a double register which holds the 0.0 value.

A64: Add and use a double register which holds the 0.0 value.

This patch also modify the crankshaft allocatable double registers because
we need this register to be callee saved to be efficient.

R=jochen@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=19803

[baptiste.afsa1, 2014-03-10 10:51:23]

PTAL

[jochen (gone - plz use gerrit), 2014-03-10 15:19:08]

lgtm

[baptiste.afsa1, 2014-03-11 15:03:44]

Committed patchset #1 manually as r19803 (presubmit successful).