tune-core2.inc: set X86ARCH32 to i686 (instead of i586)

Use i686 as TARGET_ARCH for 32bit core2 (and corei7 and atom) builds.

In most cases, i586 and i686 are equivalent values for TARGET_ARCH, however
one important exception is glibc. When configured for i686, glibc enables
optimised string functions (SSE, SSE2, etc), which are not used when
building for i586.

The benefits of i686 optimised string functions vary depending on the
application and the CPU, however in some cases the improvements are
significant. In one test, a 50% increase in FPS was seen when running the
'smashcat' benchmark [1] in a qtwebkit browser on an Intel Atom based SoC.
The gain seems to comes from a 3x improvement in memcpy performance when
copying graphics buffer lines (5120 bytes, or 1280 x 4 bytes/pixel), from
the CPU to GPU. Note that very large memcpy's (e.g. 32MB) on the same
machine show no particular performance increase between i586 and i686.

  [1] http://www.smashcat.org/av/canvas_test/

Warning: The change in TARGET_ARCH means that _i586 architecture specific
over-rides will no longer take effect. Both oe-core and meta-oe have been
updated to replace _i586 over-rides with _x86, however other layers may
still need review and updating.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>

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