consolidate libc and libssp into compiler-rt #7265
Description
Right now, zig generates a "c" library that has functions such as memcpy and sqrt because LLVM generates code that on those symbols. I propose to make zig's compiler-rt have all the symbols that codegen depends on, regardless of what library will typically provide them. Then our existing trick of using weak exports (which can be overridden by e.g. linking against libc) will make sure we don't provide too many functions. We already use function-sections to omit unused symbols.
The benefits of this are simplicity, which will become even more clear when we have 2 competing backends: the LLVM one and the self-hosted zig one, which will each require a different set of compiler-rt symbols. We may even want to expose a build option to compiler-rt.zig (or maybe a different entry point?) so that compiler-rt source can know which backend it needs to provide symbols for.
The simplicity is not only for zig compiler developers, but for users as well. For example, this would simplify the usage of #7264 - we would not need equivalent CLI flags for these other 2 libs, and users would not need to deal with all 3 things.