toolchain/gnu: also define NM

llvm is defining NM already and `openthread` is using the symbol.

makefiles/toolchain/gnu.inc.mk

@@ -9,6 +9,7 @@ export AR         = $(PREFIX)ar
 export RANLIB     = $(PREFIX)ranlib
 endif
 export AS         = $(PREFIX)as
+export NM         = $(PREFIX)nm
 export LINK       = $(PREFIX)gcc
 export LINKXX     = $(PREFIX)g++
 export SIZE       = $(PREFIX)size