Don't generate (S)SSE3 instructions in static code.

-march=core2 implies support for both the SSE3 and SSSE3 instruction
set extensions. Chrome should run on CPUs with only SSE2 as well.

Also, make use of SSE on x86-32 for floating-point operations, to be
consistent with x86-64, and don't favor any specific architecture for
tuning.

Bug chromium:797763

Change-Id: Ia17428734460ed855f3aa0f83c6d2f6b3775094c
Reviewed-on: https://swiftshader-review.googlesource.com/15688
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 6403ca8..9404d0b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -74,13 +74,16 @@
       cflags += [
         "-m64",
         "-fPIC",
-        "-march=core2",
+        "-march=x86-64",
+        "-mtune=generic",
       ]
     } else {  # 32 bit version
       cflags += [
         "-m32",
         "-msse2",
-        "-march=i686",
+        "-mfpmath=sse",
+        "-march=pentium4",
+        "-mtune=generic",
       ]
     }