tune-core2: use n270 instead of core2duo

tune-core2: use n270 instead of core2duo

Fixes warnings in builds using core2-32 tune:

warning: TCG doesn't support requested feature: CPUID.80000001H:EDX.syscall [bit 11]
warning: TCG doesn't support requested feature: CPUID.80000001H:EDX.lm [bit 29]

when executing postinsts using qemu-i386.

i386 target doesn't enable CPUID_EXT2_SYSCALL and CPUID_EXT2_LM [1]
while cpu choice of core2duo that we use for core2-32 TUNE does [2].
Use n270 cpu instead to use with qemu which supports SSSE3 and doesn't
have these bits enabled [3].

[1] https://github.com/qemu/qemu/blob/master/target/i386/cpu.c#L739
[2] https://github.com/qemu/qemu/blob/master/target/i386/cpu.c#L1439
[3] https://github.com/qemu/qemu/blob/master/target/i386/cpu.c#L1603

Fixes [YOCTO #12916]

Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>