init: shaext is supported in openssl and better on ryzen than b2

src/borg/archiver.py

@@ -2452,7 +2452,8 @@ class Archiver:
 
         On modern Intel/AMD CPUs (except very cheap ones), AES is usually
         hardware-accelerated.
-        BLAKE2b is faster than SHA256 on Intel/AMD 64-bit CPUs,
+        BLAKE2b is faster than SHA256 on Intel/AMD 64-bit CPUs
+        (except AMD Ryzen and future CPUs with SHA extensions),
         which makes `authenticated-blake2` faster than `none` and `authenticated`.
 
         On modern ARM CPUs, NEON provides hardware acceleration for SHA256 making it faster