[WIP] toolchain: Add cxx_lib attribute to inject libc++ in x-compile

[phlax]

when cross-compiling you only need the exec toolchain and not the target one, except for the c++ libs themselves

to work around this currently toolchains_llvm falls back to using libstdc++ on the assumption that it is more likely to be present either on the host or in the sysroot

these foreign arch libs are only provided in the huge arch-specific toolchain or release tarballs, so we were keen not to have consumers downloading multiple toolchains for additional arches

to workaround this, we (envoy) now publish tarballs containining only lib/ and include/ folders with the libc++ libs fished out of the llvm release tarball

this PR allows toolchains_llvm to make use of this - example config from current envoy (workspace):

load("@envoy_toolshed//compile:libcxx_libs.bzl", "setup_libcxx_libs")
setup_libcxx_libs()

load("@toolchains_llvm//toolchain:rules.bzl", "llvm_toolchain")

llvm_toolchain(
    name = "llvm_toolchain",
    llvm_version = "18.1.8",
    # These libs are only included for cross-compile targets
    cxx_lib = {
        "linux-aarch64": "@libcxx_libs_aarch64",
        "linux-x86_64": "@libcxx_libs_x86_64",
    },
    cxx_standard = {"": "c++20"},
    sysroot = {} if USE_LOCAL_SYSROOT else {
        "linux-x86_64": "@sysroot_linux_amd64//:sysroot",
        "linux-aarch64": "@sysroot_linux_arm64//:sysroot",
    },
    toolchain_roots = {"": LLVM_PATH} if LLVM_PATH else {},
)
    

it can also be used with bzlmod like so

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm", dev_dependency=True)
llvm.cxx_lib(
    name = "llvm_toolchain",
    label = "@libcxx_libs_aarch64",
    targets = ["linux-aarch64"],
)
llvm.cxx_lib(
    name = "llvm_toolchain",
    label = "@libcxx_libs_x86_64",
    targets = ["linux-x86_64"],
)
llvm.toolchain(
    name = "llvm_toolchain",
    llvm_version = "18.1.8",
)
use_repo(llvm, "llvm_toolchain")

partial fix for #694

[dzbarsky]

What issues did you hit? In my experience, the c++ standard library include/ doesn't vary between targets (aside from a few minor things like fortran and libomp headers) and when cross-compiling the cxx lib comes from the sysroot, which this toolchain already supports?

[phlax]

i didnt really want to put it into sysroot, our sysroot currently does have libstdc++ but i wanted to remove that also - so its just glibc

there is a fix that this pr requires, but otherwise this is working really nicely in patches

[phlax]

@dzbarsky i probably didnt answer your question very well - i have updated the description which should hopefully address it

if including this is something the project is willing to consider, i would be happy to add some tests/docs

[phlax]

fwiw - i think cxx_lib -> cxx_cross_lib to make it clear its only used for cross compile

more fixes from downstream testing being tested atm - will update once that is done