Use SSE 4.2 as a baseline when compiling Godot

This lets the compiler do more optimizations, leading to increased
performance for demanding CPU tasks.

SConstruct

@@ -731,11 +731,25 @@ elif env.msvc:
         )
         Exit(255)
 
-# Default architecture flags.
-if env["arch"] == "x86_32":
-    if env.msvc:
+# Set x86 CPU instruction sets to use by the compiler's autovectorization.
+if env["arch"] == "x86_64":
+    # On 64-bit x86, enable SSE 4.2 and prior instruction sets (SSE3/SSSE3/SSE4/SSE4.1) to improve performance.
+    # This is supported on most CPUs released after 2009-2011 (Intel Nehalem, AMD Bulldozer).
+    # AVX and AVX2 aren't enabled because they aren't available on more recent low-end Intel CPUs.
+    if env.msvc and not methods.using_clang(env):
+        # https://stackoverflow.com/questions/64053597/how-do-i-enable-sse4-1-and-sse3-but-not-avx-in-msvc/69328426
+        env.Append(CCFLAGS=["/d2archSSE42"])
+    else:
+        # `-msse2` is implied when compiling for x86_64.
+        env.Append(CCFLAGS=["-msse4.2"])
+elif env["arch"] == "x86_32":
+    # Be more conservative with instruction sets on 32-bit x86 to improve compatibility.
+    # SSE and SSE2 are present on all CPUs that support 64-bit, even if running a 32-bit OS.
+    if env.msvc and not methods.using_clang(env):
         env.Append(CCFLAGS=["/arch:SSE2"])
     else:
+        # Use `-mfpmath=sse` to use SSE for floating-point math, which is more stable than x87.
+        # `-mstackrealign` is needed for it to work.
         env.Append(CCFLAGS=["-msse2", "-mfpmath=sse", "-mstackrealign"])
 
 # Explicitly specify colored output.