[WebAssembly] Add cbrt function signatures

Summary:
Fixes a crash in the backend where optimizations produce calls to the
cbrt runtime functions. Fixes PR 44227.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74259