Per cpu code simplification V2

This patchset simplifies the code that arches need to maintain to support
per cpu functionality. Most of the code is moved into arch independent
code. Only a minimal set of definitions is kept for each arch.

The patch also unifies the x86 arch so that there is only a single
asm-x86/percpu.h

V1->V2:
- Add support for specifying attributes for per cpu declarations (preserves
  IA64 model(small) attribute).
- Drop first patch that removes the model(small) attribute for IA64
- Missing #endif in powerpc generic config /  Wrong Kconfig
- Follow Randy's suggestions on how to do the Kconfig settings
- Rediff against 2.6.24-rc3-mm2

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