powerpc/uaccess: Implement masked user access #234

Job	Run time
sparse (ppc64, fedora-40, ppc64)	31m 42s
sparse (pmac32, fedora-40, ppc64)	14m 47s
sparse (mpc885_ads_defconfig, fedora-40, ppc64)	8m 20s
sparse (ppc64le, ppc64le, fedora-40)	31m 2s
	1h 25m 51s

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