linker: ensure global constructors only run once

Rename the symbols used to denote the locations of the global
constructor lists and modify the Zephyr start-up code accordingly.
On POSIX systems this ensures that the native libc init code won't
find any constructors to run before Zephyr loads.

Fixes #39347, #36858

Signed-off-by: David Palchak <palchak@google.com>