Support hardware SHA-256 and SHA-1 on Arm platforms.

Similarly to my recent addition of NEON-accelerated AES, these new
implementations drop in alongside the SHA-NI ones, under a different
set of ifdefs. All the details of selection and detection are
essentially the same as they were for the AES code.

ssh.h

@@ -856,6 +856,8 @@ extern const ssh_compression_alg ssh_zlib;
  * platform subdirectory.
  */
 bool platform_aes_hw_available(void);
+bool platform_sha256_hw_available(void);
+bool platform_sha1_hw_available(void);
 
 /*
  * PuTTY version number formatted as an SSH version string.