Fix multiple compiler warnings when building with gcc-12

[John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>]

This patch series fixes a number of warnings when building powerpc-utils
with gcc-12 or newer. Since the project builds with "-Werror" by default,
these warnings will cause the build to fail.

With these patches applied, all warnings are gone when building on ppc64el,
there are two additional warnings left regarding possibly truncated strings
when building on big-endian PowerPC targets for which I will send a separate
patch set.

Cheers,
Adrian

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