SCons: Properly set SSE2 as baseline on x86_32

Setting it only for release templates on Windows and macOS was inconsistent,
and Jolt requires it as a minimum.

Drop the `-mxsave` flag from the raycast module, this doesn't seem to be
used explicitly by Embree, and unnecessarily makes our config and baseline
muddy.

SConstruct

@@ -713,6 +713,12 @@ elif env.msvc:
         )
         Exit(255)
 
+# Default architecture flags.
+if env["arch"] == "x86_32":
+    if env.msvc:
+        env.Append(CCFLAGS=["/arch:SSE2"])
+    else:
+        env.Append(CCFLAGS=["-msse2"])
 
 # Set optimize and debug_symbols flags.
 # "custom" means do nothing and let users set their own optimization flags.