bootstrap: Add Apple Silicon support, fix Darwin URL typo

The existing Darwin URL fetches the x86_64 toolchain, which fails
on Apple Silicon hosts under Rosetta due to hardcoded Intel-Homebrew
dylib paths in cc1. Add Darwin-arm64 URL for ARM's native arm64
build; bootstrap.py's per-architecture selection logic picks it up
automatically.

Also fix a typo in the existing Darwin URL (.tar.xzg -> .tar.xz).
The typo currently works only because ARM's servers silently
redirect to the canonical file; relying on that is brittle.

Author: boinger

utils/bootstrap.py

@@ -59,7 +59,8 @@
             'Linux': 'https://developer.arm.com/-/media/Files/downloads/gnu/13.2.rel1/binrel/arm-gnu-toolchain-13.2.rel1-x86_64-arm-none-eabi.tar.xz',
             'Linux-aarch64': 'https://developer.arm.com/-/media/Files/downloads/gnu/13.2.rel1/binrel/arm-gnu-toolchain-13.2.rel1-aarch64-arm-none-eabi.tar.xz',
             'Windows': 'https://developer.arm.com/-/media/Files/downloads/gnu/13.2.rel1/binrel/arm-gnu-toolchain-13.2.rel1-mingw-w64-i686-arm-none-eabi.zip',
-            'Darwin': 'https://developer.arm.com/-/media/Files/downloads/gnu/13.2.rel1/binrel/arm-gnu-toolchain-13.2.rel1-darwin-x86_64-arm-none-eabi.tar.xzg',
+            'Darwin': 'https://developer.arm.com/-/media/Files/downloads/gnu/13.2.rel1/binrel/arm-gnu-toolchain-13.2.rel1-darwin-x86_64-arm-none-eabi.tar.xz',
+            'Darwin-arm64': 'https://developer.arm.com/-/media/Files/downloads/gnu/13.2.rel1/binrel/arm-gnu-toolchain-13.2.rel1-darwin-arm64-arm-none-eabi.tar.xz',
         }
     },
     'clang-format': {