Avoid LLVM API for setting a module's code model and PIC/PIE levels

[alexrp]

See: https://github.com/ziglang/zig/pull/21224/files#r1734518792

We should be able to do this with our own bitcode writer just fine as these are just module flags.

[jacobly0]

This is a bug because if you want to use the emitted bitcode directly it will have the incorrect module flags as ZigLLVMSetModule* won't have been called yet.