sha2: use cpufeatures to detect sha2 on aarch64

[tarcieri]

Uses the newly added aarch64 support in the cpufeatures crate for sha2 CPU feature detection on Linux and macOS/M1.

[tarcieri]

Confirm that the tests pass and this successfully detects sha2 support on an M1 Mac Mini:

test bench1_10    ... bench:           6 ns/iter (+/- 0) = 1666 MB/s
test bench2_100   ... bench:          47 ns/iter (+/- 2) = 2127 MB/s
test bench3_1000  ... bench:         463 ns/iter (+/- 2) = 2159 MB/s
test bench4_10000 ... bench:       4,639 ns/iter (+/- 30) = 2155 MB/s

[tarcieri]

Unfortunately it doesn't work on Linux because the cpufeatures macro is expanding code containing references to libc, which is a bug.

I will go ahead and extract that code into a small helper function ala the MacOS implementation.

Edit: fix here RustCrypto/utils#398