mc: Reduce stack use in {put,prep}_scaled_{bilin,8tap}

For the bilin cases, this seems to make things marginally faster
(measured on x86_64; 7-25% faster with compiler autovectorization).
For 8tap, it doesn't make much of a difference at all.

Before:                                      GCC   Clang
mc_scaled_8tap_regular_w128_8bpc_c:     115155.5   98549.3
mc_scaled_8tap_regular_w128_8bpc_ssse3:  17936.0   18411.1
mc_scaled_bilinear_w128_8bpc_c:          40290.0   51812.9
mc_scaled_bilinear_w128_8bpc_ssse3:      18243.9   18177.0
After:
mc_scaled_8tap_regular_w128_8bpc_c:     116304.3   99453.2
mc_scaled_8tap_regular_w128_8bpc_ssse3:  18387.0   18077.3
mc_scaled_bilinear_w128_8bpc_c:          37381.4   41145.0
mc_scaled_bilinear_w128_8bpc_ssse3:      18423.8   18031.6

(Benchmarked with the seed 0; the total runtime for the scaled
benchmarks are significantly affected by the random seed.)

This reduces the stack usage of these functions from around 65 KB
each, to less than 1 KB for bilin, and around 2 KB for 8tap.

With this in place, the required stack space for dav1d should
be mostly identical across configurations; on x86_64 (both with
and without assembly), it can run with 62 KB of stack, and
on arm and aarch64, it can run with 58 KB of stack.