Apple Silicon support

[tmcdonell]

Update formula to support building on apple silicon. Not sure how far back we can go for this...

[bucanero]

I was able to manually build llvm-10 on M1 from sources. My build is not the "full" build, but I only needed it as a cross-compiler to generate PS4 (x86) code, and it worked just fine.

Just for the record, here's how I built it: (cmake and ninja are required, you can install them with brew install cmake ninja)

Download and unpack the v10.0.1 source files:
https://github.com/llvm/llvm-project/releases/download/llvmorg-10.0.1/llvm-project-10.0.1.tar.xz

create a folder for this build

cd llvm-project-10.0.1
mkdir build
cd build

call cmake to generate the required build scripts:

cmake -G Ninja \
    -DDEFAULT_SYSROOT="$(xcrun --show-sdk-path)" \
    -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi;lld;clang-tools-extra" \
    -DLLVM_TARGETS_TO_BUILD=all -DCMAKE_BUILD_TYPE=Release ../llvm

compile:

ninja clang
ninja cxx
ninja lld
ninja llvm-ar

when finished, you'll have the llvm 10 native binaries in the bin folder.
You can copy the build folder to /usr/local/opt/llvm and use it as needed. (in my case, with the Open Orbis SDK)