aarch64_cpudetect: 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).

Based on patch from Martin Storsjö on ffmpeg-devel:
https://ffmpeg.org/pipermail/ffmpeg-devel/2024-September/333609.html

Change-Id: I77117bee8516924fddcdecccae8bab3cf5beed96

vpx_ports/aarch64_cpudetect.c

@@ -78,7 +78,18 @@ static int arm_get_cpu_caps(void) {
   }
 #endif  // defined(PF_ARM_V82_DP_INSTRUCTIONS_AVAILABLE)
 #endif  // HAVE_NEON_DOTPROD
-  // No I8MM or SVE feature detection available on Windows at time of writing.
+#if HAVE_NEON_I8MM
+// Support for PF_ARM_SVE_I8MM_INSTRUCTIONS_AVAILABLE was added in Windows SDK
+// 26100.
+#if defined(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 |= HAS_NEON_I8MM;
+  }
+#endif  // defined(PF_ARM_SVE_I8MM_INSTRUCTIONS_AVAILABLE)
+#endif  // HAVE_NEON_I8MM
   return flags;
 }