Use libc++ for x86_64 toolchain

[LittleHuba]

This enables the rust toolchain to compile with the llvm toolchain as backend.

[pawelrutkaq]

helo @LittleHuba @castler . You have broken S-CORE toochain without asking anyone to review. We will revert the change.
Please if your repo require llvm use extensions in repo.

ferrocene = use_extension(
    "//extensions:ferrocene_toolchain_ext.bzl",
    "ferrocene_toolchain_ext",
)

to creeat toolchain