ARM: Fix register misuse bug in Allocate().

The bug is triggered if flags contains DOUBLE_ALIGNMENT and the
object_size is not an ARM immediate value. In this case, the code
for DOUBLE_ALIGNMENT uses the scratch2 register, which is aliased
to obj_size_reg containing the object_size.

Instead of pre-loading the object_size, which is difficult since
we are out of registers here, we simply generate a non-empty
sequence of add instructions for the addition of the constant
object_size (carefully handling possible overflow in each step).

Also turn static ASSERT into STATIC_ASSERT in Allocate().

BUG=v8:2851
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23323002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16221 ce2b1a6d-e550-0410-aec6-3dcde31c8c00