make _sbss override-able, add a fictitious .stack section

This is one possible solution to the stack overflow problem described in #34. This approach uses a
linker wrapper, called [swap-ld], to generate the desired memory layout. See #34 for a description
of the desired memory layout and #41 for a description of how `swap-ld` works.

The observable effects of this change in cortex-m programs are:

- the `_sbss` symbol is now override-able.
- there is now a `.stack` linker section that denotes the span of the call stack. `.stack` won't be
  loaded into the program; it just exists for informative purposes (`swap-ld` uses this
  information).

Given the following program:

``` rust
fn main() {
    static mut X: u32 = 0;
    static mut Y: u32 = 1;

    loop {
        unsafe {
            ptr::write_volatile(&mut X, X + 1);
            ptr::write_volatile(&mut Y, Y + 1);
        }
    }
}
```

If you link this program using the `arm-none-eabi-ld` linker, which is the cortex-m-quickstart
default, you'll get the following memory layout:

``` console
$ console
section                                                                   size         addr
.vector_table                                                            0x130    0x8000000
.text                                                                     0x94    0x8000130
.rodata                                                                    0x0    0x80001c4
.stack                                                                  0x5000   0x20000000
.bss                                                                       0x4   0x20000000
.data                                                                      0x4   0x20000004
```

Note how the space reserved for the stack (depicted by the `.stack` linker section) overlaps with
the space where .bss and .data reside.

If you, instead, link this program using `swap-ld` you'll get the following memory layout:

``` console
$ arm-none-eabi-size -Ax app
section                                                                   size         addr
.vector_table                                                            0x130    0x8000000
.text                                                                     0x94    0x8000130
.rodata                                                                    0x0    0x80001c4
.stack                                                                  0x4ff8   0x20000000
.bss                                                                       0x4   0x20004ff8
.data                                                                      0x4   0x20004ffc
```

Note that no overlap exists in this case and that the call stack size has reduced to accommodate the
.bss and .data sections.

Unlike #41 the addresses of static variables is now correct:

``` console
$ arm-none-eabi-objdump -CD app
Disassembly of section .vector_table:

08000000 <_svector_table>:
 8000000:       20004ff8        strdcs  r4, [r0], -r8 ; initial Stack Pointer

08000004 <cortex_m_rt::RESET_VECTOR>:
 8000004:       08000131        stmdaeq r0, {r0, r4, r5, r8}

08000008 <EXCEPTIONS>:
 8000008:       080001bd        stmdaeq r0, {r0, r2, r3, r4, r5, r7, r8}
 (..)

Disassembly of section .stack:

20000000 <.stack>:
        ...

Disassembly of section .bss:

20004ff8 <cortex_m_quickstart::main::X>:
20004ff8:       00000000        andeq   r0, r0, r0

Disassembly of section .data:

20004ffc <_sdata>:
20004ffc:       00000001        andeq   r0, r0, r1
```

closes #34

[swap-ld]: https://github.com/japaric/swap-ld

Author: japaric

cortex-m-rt/link.x

@@ -48,9 +48,19 @@ SECTIONS
     . = ALIGN(4);
   } > FLASH
 
-  .bss : ALIGN(4)
+  /* limits of the .stack region */
+  _sstack = ORIGIN(RAM);
+  _estack = _stack_start;
+
+  /* ficticious region that represents the memory available for the stack */
+  .stack _sstack (INFO) : ALIGN(4)
+  {
+    . += (_estack - _sstack);
+  }
+
+  PROVIDE(_sbss = ORIGIN(RAM));
+  .bss _sbss : ALIGN(4)
   {
-    _sbss = .;
     *(.bss .bss.*);
     . = ALIGN(4);
     _ebss = .;