Previously these were just using the raw function name, which led
to markers like "Slab_begin". Now we prefix RANGE_MARKER_ so there
is absolutely no chance of confusion, and the symbols are clearly
visible in dumps.
Also add a note that the kernel must be built with -fno-toplevel-reorder
for these to work. (It seems when this was implemented, GCC had not yet
implemented top-level reordering.)
They are only used for debugging with the tracing system in a handful
of places, and -ftoplevel-reorder is enabled with optimizations for
a reason, so it makes more sense just to note this and not to enable
that option by default (i.e. in the off chance someone will want to
use these in non-debug builds, like I did.)
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