First program linking issues #52

Hi,

I am following the Copper book, and I am currently having problems with the very first program (http://japaric.github.io/copper/first/build.html).

The program compiles, but the objdump output is the following:

$arm-none-eabi-objdump -CD target/thumbv7m-none-eabi/debug/app

target/thumbv7m-none-eabi/debug/app:     file format elf32-littlearm


Disassembly of section .text:

00000000 <_reset-0x8>:
   0:	20010000 	andcs	r0, r1, r0
   4:	00000009 	andeq	r0, r0, r9

For some reason the main function is optimized away at some point.
If I make rustc emit the assembly file, I can see the function is there, with the right symbol (_reset), so I guess that the problem comes while linking.