x86: simplify ceil(log2(x)) function

[dougm]

Summary

A function called mask_width in one place and log2 in the other calculates its value in a more complex way than necessary. A simpler implementation offered here saves a few bytes in the functions that call it. 128 bytes in add_deterministic_cache, in particular.

[avg]

Looks good to me.
A bit ashamed that I could not figure that fls(x - 1) does the job if zero is handled as a special case.

[alc]

sys/amd64/vmm/x86.c

63–64

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