SCons: Explicitly enable -mfpmath=sse -mstackrealign for x86_32

Passing `-msse2` doesn't seem to be sufficient to opt into SSE floating point math
instead of the less stable x87.

`-mstackrealign` also seems necessary when using SSE on x86_32.

SConstruct

@@ -718,7 +718,7 @@ if env["arch"] == "x86_32":
     if env.msvc:
         env.Append(CCFLAGS=["/arch:SSE2"])
     else:
-        env.Append(CCFLAGS=["-msse2"])
+        env.Append(CCFLAGS=["-msse2", "-mfpmath=sse", "-mstackrealign"])
 
 # Explicitly specify colored output.
 if methods.using_gcc(env):