Double-align Zone allocations to fix MIPS issue.

Double-align Zone allocations to fix MIPS issue.

Also ensure initial buffer is aligned.

~2% increase in total allocated zone memory on compile-heavy benchmarks.
No significant impact on speed for either ia32 or arm.

R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=42797

[Ivan Posva, 2015-01-13 00:07:24]

LGTM -ip

[koda, 2015-01-13 01:56:34]

Committed patchset #2 (id:20001) manually as r42797 (presubmit successful).

[zra, 2015-01-14 15:44:20]

zra@google.com changed reviewers:
+ zra@google.com

[zra, 2015-01-14 15:44:20]

lgtm