scripts: Replaced nm with objdump in code.py/data.py

There is an argument for prefering nm for code size measurements due to
portability. But I'm not sure this really holds up these days with
objdump being so prevalent.

We already depend on objdump for ctx/structs/perf and other dwarf info,
so using objdump -t to get symbol information means one less tool to
depend on/pass around when cross-compiling.

As a minor benefit this also gives us more control over which sections
to include, instead of relying on nm's predefined t/r/d/b section types.

---

Note code.py/data.py did _not_ require objdump before this. They did use
objdump to map symbols to source files, but would just guess if
objdump wasn't available.

Makefile

@@ -64,7 +64,6 @@ CC            ?= gcc
 AR            ?= ar
 SIZE          ?= size
 CTAGS         ?= ctags
-NM            ?= nm
 OBJDUMP       ?= objdump
 VALGRIND      ?= valgrind
 GDB           ?= gdb
@@ -115,10 +114,6 @@ endif
 # forward -j flag
 PERFFLAGS   += $(filter -j%,$(MAKEFLAGS))
 PERFBDFLAGS += $(filter -j%,$(MAKEFLAGS))
-ifneq ($(NM),nm)
-CODEFLAGS += --nm-path="$(NM)"
-DATAFLAGS += --nm-path="$(NM)"
-endif
 ifneq ($(OBJDUMP),objdump)
 CODEFLAGS    += --objdump-path="$(OBJDUMP)"
 DATAFLAGS    += --objdump-path="$(OBJDUMP)"