ppc: Define SETFIELD for the ppc target

It keeps repeating, move it to the header. This uses __builtin_ffsl() to
allow using the macros in #define.

This is not using the QEMU's FIELD macros as this would require changing
all such macros found in skiboot (the PPC PowerNV firmware).

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
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Changes:
v2:
* preserved the comment about skiboot
* copied the actual macros from skiboot:
https://github.com/open-power/skiboot/blob/master/include/bitutils.h#L31
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