configure.ac: use AC_PATH_TOOL for nm

[nwf]

nm might, especially in cross-compilation scenarios, be available but prefixed with the target triple. So: use AC_PATH_TOOL rather than AC_PATH_PROG to find it. (See https://www.gnu.org/software/autoconf/manual/autoconf-2.68/html_node/Generic-Programs.html .)

Found and proposed fix tested by cross-compiling Asterisk using Nixpkgs on x86_64 targeting aarch64. :)

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Attention! This pull request may contain issues that could prevent it from being accepted. Please review the checklist below and take the recommended action. If you believe any of these are not applicable, just add a comment and let us know.

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[jcolp]

The Asterisk project commits a generated configure script to make it easy for users. You will need to regenerate using bootstrap.sh and include the regenerated configure script in this change.

[jcolp]

The PR check also needs to be fulfilled and branches specified.

[nwf]

Running ./bootstrap.sh on 5d15a42 makes changes to ./configure. I've pulled those out to a commit prior to my change, and then ran ./bootstrap.sh again after AC_PATH_TOOL-ization of nm as a 3rd commit. Please feel free to squash appropriately, or tell me that I have done something wrong because the first of those should be empty.

As to cherry-picking:
cherry-pick-to: 23
cherry-pick-to: 22
cherry-pick-to: 20

[jcolp]

Squash it all down to one commit, it's fine.

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