aarch64: Detect I8MM on Windows via SVE-I8MM

There's no direct processor feature constant for I8MM alone, but
there is a flag for SVE-I8MM (added in WinSDK 10.0.26100 and
recent versions of mingw-w64). If SVE-I8MM is available, we can
assume that I8MM is available.

While HW supporting these features isn't yet commonly running
Windows, this at least allows detecting and running the I8MM codepaths
in Windows builds in Wine (possibly running in QEMU).

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/aarch64/cpu.c

@@ -112,6 +112,13 @@ static int detect_flags(void)
 #ifdef PF_ARM_V82_DP_INSTRUCTIONS_AVAILABLE
     if (IsProcessorFeaturePresent(PF_ARM_V82_DP_INSTRUCTIONS_AVAILABLE))
         flags |= AV_CPU_FLAG_DOTPROD;
+#endif
+#ifdef PF_ARM_SVE_I8MM_INSTRUCTIONS_AVAILABLE
+    /* There's no PF_* flag that indicates whether plain I8MM is available
+     * or not. But if SVE_I8MM is available, that also implies that
+     * regular I8MM is available. */
+    if (IsProcessorFeaturePresent(PF_ARM_SVE_I8MM_INSTRUCTIONS_AVAILABLE))
+        flags |= AV_CPU_FLAG_I8MM;
 #endif
     return flags;
 }