ncg/aarch64: Don't use x18 register on AArch64/Darwin

Apple's ABI documentation [1] says: "The platforms reserve register x18.
Don’t use this register." While this wasn't problematic in previous
Darwin releases, macOS 13 appears to start zeroing this register
periodically. See #21964.

[1] https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms

(cherry picked from commit 67575f20)