We don't enable aarch64 build by default yet. This will be done in a
second part (tor-browser-build#40439) 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:
tpo/applications/tor-browser-build#40158 (comment 2836499)