Implement ELF toolchain based on GNU toolchain codes

[atirut-w]

This allows Clang to directly use z80-elf binutils toolchain for assembling and linking.

$ ./bin/clang -target z80-elf -nostartfiles -nostdlib test.c
/usr/bin/z80-elf-ld: warning: cannot find entry symbol _start; defaulting to 00000100

(The warning is also present when assembling and linking manually with the binutils toolchain)

Note

The standard executable linking with runtime object files can't really be tested without a complete Z80 sysroot, and I am not sure how to build one. Right now, you need to use -nostartfiles -nostdlib to compile executables. The PR works completely fine otherwise.

[adriweb]

Does this take over the default "mode of operation" so far, especially for eZ80? Or does it have to be triggered manually.
Because most importantly we shouldn't break the current behavior of the ez80-clang at least.

[atirut-w]

Ah, that's something to fix, then. The toolchain driver does the following so that GAS doesn't error out:

void Z80ToolChain::addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
                                        llvm::opt::ArgStringList &CC1Args,
                                        Action::OffloadKind) const {
  // Add -mllvm --z80-gas-style by default
  CC1Args.push_back("-mllvm");
  CC1Args.push_back("--z80-gas-style");
}

Any idea on how I should make it backward-compatible?

[adriweb]

Not sure how to do that, but somehow trigger all the new things only if there's some specific option?

[atirut-w]

That means you'd need another flag to make Clang compile, assemble, and link in one step. It should be as easy as a quick one-line edit, but I'm not sure about the UX.

[adriweb]

Well, considering this repo is intended for the CE toolchain primarily, it seems like the good choice until the CE toolchain adopts a GNU-toolchain-compatibility thing by default.
Then the old (current) behavior can become the one with a flag instead, I guess.

[atirut-w]

I'll do that real quick then

[atirut-w]

Okay, it seems I can't just add a new option as simply as adding a new global for Clang. I guess having the user manually add -mllvm --z80-gas-style is fine for now?