OS X build fixes

Changes in Rust 0.11-pre may have necessitated the attribute
modifications in common/lib.rs
Fix bare raw pointers, added offset(*mut T) + docs

Left notes where I wasn’t 100% certain that the pointer should be
mut/const.
Print the SP and IP in hex
Update README.md
Fix Makefile errors when LD/etc already defined
Remove some c_types dependency
Use tmp file with sed for OS X/BSD compatibility
Merge branch 'master' of https://github.com/alexchandel/rustboot
Merge pull request #12 from alexchandel/master

OS X build fixes