fusion: don't let a broken llvm-nm silently drop the symbol prefix

[atassis]

Summary

KernelCompilationRule._prefix_symbols builds a redefine-syms map with
nm --defined-only --extern-only obj | awk '...' > map and applies it with
objcopy --redefine-syms=map. If nm fails, the map comes out empty, objcopy renames nothing, and every
fused kernel object keeps its bare symbol, which surfaces only much later as undefined symbol: <prefix><sym>
at the per-core link. Two things make that failure silent:

1. _find_tool returns the first existing llvm-nm / llvm-objcopy even if that binary cannot actually
   run (for example one with an unsatisfied shared-library dependency).
2. In nm | awk > map the pipeline's exit status is awk's, so a failure of nm is masked.

Fix

• Add _find_working_tool, which probes each candidate with --version and skips a present-but-unrunnable
  binary, falling through to the system PATH copy and finally to _find_tool (so the existing not-found
  error is preserved). _prefix_symbols and _rename_symbols use it to select llvm-nm / llvm-objcopy.
• Run nm to a file joined to awk with &&, so a failing nm aborts the build loudly instead of
  yielding an empty map.

Both changes are no-ops when the tools work: the same binary is selected and the same map is produced.

Testing

Reproduced the original silent failure by selecting an llvm-nm that exits nonzero; before the change the
fused link failed with undefined symbol: <prefix><sym>, and after it the build either succeeds (a working
nm is found on PATH) or aborts at the map step with nm's own error, instead of failing opaquely at link
time. No behavior change when nm / objcopy run normally.