powerpc: Drop version_string placement optimization

powerpc: Drop version_string placement optimization

As explained by Wolfgang, historically PowerPC would do a number of
things to hand-optimize placement of the binary on NOR flash in order to
maximize utilization of very scarce resources.  These days, we simply
aren't optimizing our binary layout for NOR flash placement and it's
quite likely this wasn't working as intended.  Furthermore, this level
of optimization makes it difficult to have version_string be a global,
instead of a weak and overridden value, and so make more progress on
reproducible builds, which is a current concern.

Move to having PowerPC no longer store version_string in the early part
of text so that it might be part of the first page of NOR and instead
use the same declaration everyone else does.

Link: https://lore.kernel.org/r/96716.1629798400@gemini.denx.de/
Signed-off-by: Tom Rini <trini@konsulko.com>