Enable AVX-512 in matrixmultiply #675

Job	Run time
cargo-careful	2m 15s
format/nightly	13s
cross_test/s390x-unknown-linux-gnu/stable	8m 21s
Pass MSRV values to other jobs	4s
miri	9m 12s
docs/nightly	22s
clippy/stable	23s
nostd/thumbv6m-none-eabi/stable	20s
cross_test/i686-unknown-linux-gnu/stable	2m 58s
blas-msrv	1m 2s
tests/stable	2m 58s
tests/nightly	3m 32s
tests/1.87.0	3m 7s
tests/beta	2m 31s
conclusion	-1s
	37m 17s

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