scons: Eliminate the useless USE_SSE2 flag.

This option would set the -msse2 compiler flag to avoid using x87
floating point, but every 64 bit x86 CPU supports sse2, and so sse2 is
enabled by default on compilers targeting it. 32 bit x86 CPUs are
obscure enough that it's not worth preserving this marginally meaningful
option which would only matter for them.

Change-Id: Ib425c261b27f1f74300e714619787d04970065ce
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40879
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -603,13 +603,8 @@ for bdir in [ base_dir ] + extras_dir_list:
 for cb in after_sconsopts_callbacks:
     cb()
 
-sticky_vars.AddVariables(
-    BoolVariable('USE_EFENCE', 'Link with Electric Fence malloc debugger',
-                 False),
-    BoolVariable('USE_SSE2',
-                 'Compile for SSE2 (-msse2) to get IEEE FP on x86 hosts',
-                 False),
-    )
+sticky_vars.Add(BoolVariable('USE_EFENCE',
+    'Link with Electric Fence malloc debugger', False))
 
 ###################################################
 #
@@ -801,9 +796,6 @@ Build variables for {dir}:
     # Save sticky variable settings back to current variables file
     sticky_vars.Save(current_vars_file, env)
 
-    if env['USE_SSE2']:
-        env.Append(CCFLAGS=['-msse2'])
-
     env.Append(CCFLAGS='$CCFLAGS_EXTRA')
     env.Append(LINKFLAGS='$LDFLAGS_EXTRA')