Teach the asm Call fast path to use paired stores for the fixed
ExecutionContext header writes and for the caller linkage fields.
This also initializes the five reserved Value slots directly instead
of looping over them as part of the general register clear path.
That keeps the hot frame setup work closer to the actual data layout:
reserved registers are seeded with a couple of fixed stores, while the
remaining register and local slots are cleared in wider chunks.
On x86_64, keep the new explicit-offset formatting on store_pair*
and load_pair* without changing ordinary [base, index, scale]
operands into base-plus-index-plus-offset addresses. Add unit
tests covering both the paired zero-offset form and the preserved
scaled-index lowering.
fa931612e1