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 <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Nicolas Schier <[e-mail]>

scripts/check-function-names.sh

···
 	exit 1
 fi
 
-bad_symbols=$(nm "$objfile" | awk '$2 ~ /^[TtWw]$/ {print $3}' | grep -E '^(startup|exit|split|unlikely|hot|unknown)(\.|$)')
+bad_symbols=$(${NM:-nm} "$objfile" | awk '$2 ~ /^[TtWw]$/ {print $3}' | grep -E '^(startup|exit|split|unlikely|hot|unknown)(\.|$)')
 
 if [ -n "$bad_symbols" ]; then
 	echo "$bad_symbols" | while read -r sym; do