[Build] Revert to AVX only for Windows

MSVC will arbitrarily include BMI2 instructions when AVX2 is enabled
and there is no way to disable this apparently, so go back to AVX
for Windows to allow Sandy/Ivy Bridge CPU support

premake5.lua

@@ -49,7 +49,12 @@ fatalwarnings("All")
 
 -- TODO(DrChat): Find a way to disable this on other architectures.
 if ARCH ~= "ppc64" then
-  filter("architecture:x86_64")
+  filter({"architecture:x86_64", "platforms:Windows"})
+    -- Use AVX instead of AVX2 on Windows to prevent MSVC from emitting BMI2 instructions
+    -- MSVC with /arch:AVX2 generates BMI2 unconditionally, breaking Sandy/Ivy Bridge CPUs
+    vectorextensions("AVX")
+  filter({"architecture:x86_64", "platforms:not Windows"})
+    -- On non-Windows (Linux/Clang), AVX2 is safe as it can be combined with -mno-bmi2
     vectorextensions("AVX2")
   filter({})
 end