powerpc/uaccess: Implement masked user access #235

Job	Run time
kernel (corenet64_smp, fedora-40, ppc64)	18m 59s
kernel (mpc885_ads, fedora-40, ppc64)	8m 53s
kernel (pmac32, fedora-40, ppc64)	17m 52s
kernel (ppc64, fedora-40, ppc64)	33m 17s
kernel (ppc64le, ppc64le, fedora-40)	31m 27s
	1h 50m 28s

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