simd: detect and surface support for Intel SHA512 extensions

Recent Intel CPUs (starting with Arrow Lake and Lunar Lake) include new
vectorised SHA512 instructions. Detect them and make them available to
the rest of the system.

Note the internal name "sha512ext". This is to disambiguate from other
uses of "sha512".

Sponsored-by: TrueNAS
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Attila Fülöp <attila@fueloep.org>
Signed-off-by: Rob Norris <rob.norris@truenas.com>
Closes #18233

include/os/linux/kernel/linux/simd_x86.h

@@ -21,6 +21,7 @@
  */
 /*
  * Copyright (C) 2016 Gvozden Neskovic <neskovic@compeng.uni-frankfurt.de>.
+ * Copyright (c) 2026, TrueNAS.
  */
 
 /*
@@ -623,6 +624,19 @@ zfs_vpclmulqdq_available(void)
 #endif
 }
 
+/*
+ * Check if SHA512 instructions are available
+ */
+static inline boolean_t
+zfs_sha512ext_available(void)
+{
+#if defined(X86_FEATURE_SHA512)
+	return (!!boot_cpu_has(X86_FEATURE_SHA512));
+#else
+	return (B_FALSE);
+#endif
+}
+
 /*
  * Check if SHA_NI instruction set is available
  */