MIPS: Increase external array allocation header size to 8 bytes.

MIPS: Increase external array allocation header size to 8 bytes.

This fixes alignment issues on MIPS HW, found for example in mjsunit external-array.
The issue originates from r11144 (86563c3e21) which adds a 4-byte header to these arrays.
This causes problems on MIPS, where certain pointers need to be 8-byte aligned.

BUG=
TEST=mjsunit/external-array

Committed: https://code.google.com/p/v8/source/detail?r=11303

[kalmard, 2012-03-31 10:52:44]

In case you don't like having the arch-specific define in the code, here are
some more solutions that could be implemented:

-set this size to 8 bytes on all architectures. It probably won't matter.
-use something similar to alignof (like in
http://codereview.chromium.org/9702114/), possibly combined with some bitwise
magic to get the necessary size.

[danno, 2012-04-02 15:48:37]

Since this is only in D8, 64-bit aligning the external buffers on architecture
seems like the simplest solution

[kalmard, 2012-04-03 15:11:19]

On 2012/04/02 15:48:37, danno wrote:
> Since this is only in D8, 64-bit aligning the external buffers on architecture
> seems like the simplest solution

Done. Please note that this patch doesn't always change the alignment to 64-bit,
it just conserves the alignment decided by malloc.

[kalmard, 2012-04-10 15:57:18]

Rebased on r11256.

[Jakob Kummerow, 2012-04-13 08:56:37]

LGTM, landing.