kbuild: prefer ${NM} in check-function-names.sh

kbuild: prefer ${NM} in check-function-names.sh

The check-function-names.sh scripts invokes 'nm' directly and this can
be problematic during cross-compilation when the toolchain is different
from the system's default (e.g. LLVM=1).

  scripts/check-function-names.sh: nm: not found

Let's prefer the ${NM} variable which is already set by kbuild. However,
still fallback to plain 'nm' to ensure the script is still usable when
called directly.

Fixes: 93863f3f859a ("kbuild: Check for functions with ambiguous -ffunction-sections section names")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20251218175824.3122690-1-cmllamas@google.com
Signed-off-by: Nicolas Schier <nsc@kernel.org>