LGTM, with a nit about tests. Please give some time for other reviewers to respond.
Consider using ## for comments and moving to the top.
(Most newer binary utility tests use ## and some reviewers find the marker makes comments stand out.)
In BFD the logic is more complex:
I think the simplified rule should be fine.
Seems reasonable, but the llvm-nm documentation needs updating (see llvm/docs/CommandGuide/llvm-nm.rst).
It may also be worth release-noting the change in behaviour too, since it will mean current ARM users who use llvm-nm will see their mapping symbols disappear from the output, if they aren't already using --special-syms.