Update asm! documentation in unstable book
Update asm! documentation in unstable book

- Update the list of supported architectures.
- Clarify issues with LLVM's use of reserved registers.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust