Enable AVX-512 in matrixmultiply #676

Job	Run time
cross_test/i686-unknown-linux-gnu/stable	2m 2s
Pass MSRV values to other jobs	3s
cargo-careful	2m 14s
clippy/stable	21s
nostd/thumbv6m-none-eabi/stable	21s
cross_test/s390x-unknown-linux-gnu/stable	7m 43s
docs/nightly	23s
miri	9m 39s
format/nightly	15s
blas-msrv	54s
tests/stable	2m 43s
tests/nightly	3m 26s
tests/beta	2m 55s
tests/1.87.0	2m 41s
conclusion	0s
	35m 40s

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