Add link_section attribute for static and fn items

This allows for control over the section placement of static, static
mut, and fn items.  One caveat is that if a static and a static mut are
placed in the same section, the static is declared first, and the static
mut is assigned to, the generated program crashes.  For example:

#[link_section=".boot"]
static foo : uint = 0xdeadbeef;

#[link_section=".boot"]
static mut bar : uint = 0xcafebabe;

Declaring bar first would mark .bootdata as writable, preventing the
crash when bar is written to.
Add simple link_section test to exercise it
auto merge of #7958 : kemurphy/rust/link-section, r=alexcrichton

This allows for control over the section placement of static, static
mut, and fn items.  One caveat is that if a static and a static mut are
placed in the same section, the static is declared first, and the static
mut is assigned to, the generated program crashes.  For example:

#[link_section=".boot"]
static foo : uint = 0xdeadbeef;

#[link_section=".boot"]
static mut bar : uint = 0xcafebabe;

Declaring bar first would mark .bootdata as writable, preventing the
crash when bar is written to.
Allow linking against crates with #[no_std]

Previously having optional lang_items caused an assertion failure at
compile-time, and then once that was fixed there was a segfault at runtime of
using a NULL crate-map (crates with no_std)
Add the ability to turn off segmented stacks

Right now the main use case for this is that it lifts the dependency on
libmorestack, which in turn lifts the dependency on librustrt. This moves even
closer to the true "bare-metal" hello world situtation of a rust program.