[llvm] Partial support for common linkage for WebAssembly (PR #151478)

[Ondřej Čertík via llvm-commits]

certik wrote:

@MaskRay yes, common is old and should not be used for new code. However, there is still a lot of software in production that uses common. If common symbols should not be used in LLVM anymore, what do you recommend Fortran compilers to emit instead? This applies to both Flang and LFortran.

https://github.com/llvm/llvm-project/pull/151478