Floating point modulus (and probably other llvm instrs) calls libc without stack switch

[brson]

As seen in #5697, LLVM has lots of places where it will insert calls to libm directly, not switching stacks.

One of places that I recall seeing this before is the translation of the fmod instruction. Rust has floating point modulus and presumably it is doing it in the red zone.

[brson]

We can solve this with an fmod lang item.

[auroranockert]

Is this an issue with calls to compiler-rt too? Or are they compiled in some special way to avoid the issue?

[catamorphism]

Nominating for milestone 5, production-ready

[thestinger]

memcpy, memmove, memset and memcmp are also all generated. As far as I can tell, we can't prevent it from generating these by altering the IR we output.

[graydon]

We can possibly be selective about when we emit memcpy/memmov, second guessing LLVM (like, never for big things)

[graydon]

accepted for production-ready milestone

[alexcrichton]

Closing due to segmented stacks being gone (stack switches no longer a thing)