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

Review URL: https://chromiumcodereview.appspot.com/9956049

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11303 ce2b1a6d-e550-0410-aec6-3dcde31c8c00