Enable double alignment for constructed arrays.

The second of three CLs that will fully enable aligned fixed double arrays.

The goal is to keep fixed double arrays 8-byte aligned so the special MIPS
instructions for loading and storing aligned doubles values in keyed operations
can be used. Achieving this, the loads and stores in corresponding keyed
operations can be done in one instruction using "ldc1" and "sdc1".

Currently, a pair of corresponding one word load/store instructions is used
as workaround for loading and storing doubles values because double
alignment is not guaranteed.

The complete set of CLs improves NavierStokes by about 20% on MIPS.

TEST=
BUG=
Align double array backing store during compaction and mark-sweep promotion.

The first of three CLs that will fully enable aligned fixed double arrays.

The goal is to keep fixed double arrays 8-byte aligned so the special MIPS
instructions for loading and storing aligned doubles values in keyed operations
can be used. Achieving this, the loads and stores in corresponding keyed
operations can be done in one instruction using "ldc1" and "sdc1".

Currently, a pair of corresponding one word load/store instructions is used
as workaround for loading and storing doubles values because double
alignment is not guaranteed.

The complete set of CLs improves NavierStokes by about 20% on MIPS.

TEST=
BUG=
MIPS: Ensure allocations in new and old data space through runtime routines are double aligned.

The third of three CLs that will fully enable aligned fixed double arrays.

The goal is to keep fixed double arrays 8-byte aligned so the special MIPS
instructions for loading and storing aligned doubles values in keyed operations
can be used. Achieving this, the loads and stores in corresponding keyed
operations can be done in one instruction using "ldc1" and "sdc1".

Currently, a pair of corresponding one word load/store instructions is used
as workaround for loading and storing doubles values because double
alignment is not guaranteed.

The complete set of CLs improves NavierStokes by about 20% on MIPS.

TEST=
BUG=
MIPS: Enable aligned keyed stores and loads for double arrays.

This comit will be squashed in previous, so ignore the commit
message.

TEST=
BUG=