Support floats with some atomic operations

[Vexu]

Adds support for floats in @atomicLoad, @atomicStore and @atomicRmw with .Xchg, .Add and .Sub.

Support for LLVMAtomicRMWBinOpFSub and LLVMAtomicRMWBinOpFAddin LLVM c API was added a week after 9.0.0 was released so this is waiting for 9.0.1.

[andrewrk]

Support for LLVMAtomicRMWBinOpFSub and LLVMAtomicRMWBinOpFAddin LLVM c API was added a week after 9.0.0 was released so this is waiting for 9.0.1.

I propose to make it work with 9.0.0 by adding the relevant wrapper code in src/zig_llvm.cpp / src/zig_llvm.h. Then we don't have to deal with people filing bug reports when their system package managers have 9.0.0 and getting compile errors when building Zig.

[andrewrk]

Also, note the official github repo is https://github.com/llvm/llvm-project, and the commit you noted there did not make it into the release/9.x branch, so the new LLVM API won't be available until llvm 10.0.0.