Remove (most) LLVM intrinsics from the compiler

[auroranockert]

This commit is similar to the old one, and does the following

• Removes most LLVM intrinsics from the compiler
• Adds functions that optimize down to specific LLVM intrinsics to
  libcore
• Adds a pragma #[constant_arg] for constant integer arguments to intrinsics (and other functions)
• Utilize the sqrt intrinsic for f32/f64 since LLVM lowers it to an actual instruction sequence.

[auroranockert]

Once a new snapshot is made, the memory-related (memmove, memcpy and memset?) intrinsics can be removed as well, but I left them unmodified for now to simplify the transition since they are called by rustc.

[brson]

@JensNockert How do you feel about this branch now? Can it be done?

[auroranockert]

I am pretty sure it can be done, but there are some (minor) issues on Windows that I cannot debug right now.

Hopefully when I am finished moving, I can rebase it upon the other intrinsics work.