Replace llvm_asm with asm

This is more idiomatic, and has a chance of compiling on stable one day, as well as be able to target more architectures. We'll have to review the options of each asm! call eventually, currently we don't specify options to a lot of places, which means we use the strictest options available (clobber memory, clobber stack, no optimizations, etc).

Note: This does not fix the remaining issues that make redox unable to run with opt-level=0. Some of the cases that were turned into asm! will have to become global_asm! anyway some day (I thought my last global_asm! MR did, but upon further inspection it would seem my tests were incorrect.)

Depends on syscall!56 (merged)