Allow non-x86_64 host architectures

[vchuravy]

Poking about at JuliaCI/PkgEval.jl#97 today I got to the step where we instantiate a build environment using BinaryBuilder.

[ Info: Running privileged container via `sudo`, may ask for your password:
 Downloading artifact: PlatformSupport-powerpc64le-linux-gnu.v2021.1.28.x86_64-linux-musl.unpacked
  Downloaded artifact: PlatformSupport-powerpc64le-linux-gnu.v2021.1.28.x86_64-linux-musl.unpacked
 Downloading artifact: GCCBootstrap-powerpc64le-linux-gnu.v7.1.0.x86_64-linux-musl.unpacked
  Downloaded artifact: GCCBootstrap-powerpc64le-linux-gnu.v7.1.0.x86_64-linux-musl.unpacked
 Downloading artifact: LLVMBootstrap.v11.0.1.x86_64-linux-musl.unpacked
  Downloaded artifact: LLVMBootstrap.v11.0.1.x86_64-linux-musl.unpacked
 Downloading artifact: PlatformSupport-x86_64-linux-musl.v2021.1.28.x86_64-linux-musl.unpacked
  Downloaded artifact: PlatformSupport-x86_64-linux-musl.v2021.1.28.x86_64-linux-musl.unpacked
 Downloading artifact: GCCBootstrap-x86_64-linux-musl.v7.1.0.x86_64-linux-musl.unpacked
    Downloading [>                                        ]  0.0 %

Would be great if we could support more than just x86_64 as a host arch.

[giordano]

😂 😂 😂

[staticfloat]

I've made some small progress toward this in my GCC rework branch, but I still consider this pie-in-the-sky.