Switch platform arch i686 to use -march=pentium4

Before, we used -march=i686, but that does not include MMX, SSE, SSE2 support,
which Julia requires. By switching to pentium4 as base level, better optimized
code can be generated, and also linking against libjulia then work for
32bit binaries without extra compiler flags.

src/Platforms.jl

@@ -87,7 +87,7 @@ const ARCHITECTURE_FLAGS = Dict(
     # Many compiler flags are the same across clang and gcc, store those in "common"
     "common" => Dict(
         "i686" => Dict(
-            "i686" => ["-march=i686", "-mtune=generic"],
+            "i686" => ["-march=pentium4", "-mtune=generic"],
             "prescott" => ["-march=prescott", "-mtune=prescott"],
         ),
         "x86_64" => Dict(