BUILD: tools: make resolve_sym_name() return a const

BUILD: tools: make resolve_sym_name() return a const

Originally it was made to return a void* because some comparisons in the
code where it was used required a lot of casts. But now we don't need
that anymore. And having it non-const breaks the build on NetBSD 9 as
reported in issue #728.

So let's switch to const and adjust debug.c to accomodate this.