Allow explicitly specifying Linux distribution

[illicitonion]

Without this, if you want to do remote execution from a non-Linux (or different Linux) platform to a Linux platform, you need to explicitly fill in the URLs attribute.

With this, you can write something like:

llvm_toolchain(
    name = "llvm_toolchain_linux_x86_64",
    llvm_version = "14.0.0",
    exec_os = "linux",
    exec_arch = "x86_64",
    exec_linux_distribution = "ubuntu",
    exec_linux_distribution_version = "18.04",
    sysroot = {
        "linux-x86_64": "@org_chromium_sysroot_linux_x64//:sysroot",
    },
)

which will work when remote building from macOS, because it won't attempt to read the Linux distribution from /etc/os-release which doesn't exist on macOS.

[rrbutani]

Thanks for the PR! This looks good to me, just a couple of nits.

---

A comment on the new attrs: in retrospect I think it would have been nicer if we'd bundled the exec_* override attrs into a single list attr:

llvm_toolchain(
    ...,
    exec_platform = ["linux", "x86_64", "ubuntu", "18.04"],
)

llvm_toolchain(
    ...,
    exec_platform = ["darwin", "aarch64"],
)

^ would let us sidestep some of the awkward permutations that are allowed (i.e. setting linux_distribution and linux_distribution_version w/o setting exec_os to linux explicitly)

(as an aside, @illicitonion: thanks for covering these in the doc strings)

But at this point it's probably not worth making a breaking change over.

[rrbutani]

Unfortunately I think we actually only try to autodetect the host distro + version if the host platform is Linux and exec_os is not specified.

What you've described seems preferable though, IMO (see below).

[rrbutani]

To read /etc/os-release when exec_os = "linux" + the host platform is Linux (not just when exec_os = None and the host is Linux) I think we'd want:

Suggested change

	if not rctx.attr.exec_os:
	(distname, version) = _linux_dist(rctx)
	return distname, version, _arch
	if rctx.os.name == "linux":
	(distname, version) = _linux_dist(rctx)
	return distname, version, _arch

---

I don't feel strongly about whether we should do the above or change the docs on exec_linux_distribution to describe the existing behavior but we should probably do one or the other.