Previously, we would unconditionally build GDB from source for the
AArch64 toolchain. This commit makes it possible to use the system's
`gdb` binary if it supports the architecture, or `aarch64-elf-gdb` if
such a package is installed.
An `aarch64-elf-gdb` package will be available through Homebrew once
this PR is merged: https://github.com/Homebrew/homebrew-core/pull/127323