We don't enable aarch64 build by default yet. This will be done in a second part (#40439 (closed)) where we create universal x86-64/arm64 dmg files.
In projects/macosx-toolchain/build we also clean up the flags used to build compiler-rt, trying to stay close to what Mozilla is doing.
We also switch back to using Ninja, to avoid an error in compiler-rt build for aarch64: #40158 (comment 2836499)
Closes #40158 (closed).