We used to prefer openssl in this situation, which
used to make sense, but now Go gained an optimized
assembly implementation for aes-gcm on arm64 with
aes instructions:
root@q1:~/go/src/github.com/rfjakob/gocryptfs# ./gocryptfs -speed
gocryptfs v1.7.1-46-g73436d9; go-fuse v1.0.1-0.20190319092520-161a16484456; 2020-04-13 go1.14.2 linux/arm64
AES-GCM-256-OpenSSL 212.30 MB/s (selected in auto mode)
AES-GCM-256-Go 452.30 MB/s
AES-SIV-512-Go 100.25 MB/s
XChaCha20-Poly1305-Go 137.35 MB/s
https://github.com/rfjakob/gocryptfs/issues/452
Now that I have discovered golang.org/x/sys/cpu and that Go
versions below 1.6 are uncommon, there was not much useful
code left in prefer_openssl.
Merge the remains into stupidgcm.
