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)

docs/users_guide/9.2.5-notes.rst

@@ -21,6 +21,8 @@ Compiler
 
 - Fix a code generation panic when using SIMD types with unboxed sums (:ghc-ticket:`22187`).
 
+- Fix a code generation bug with the native code generator on aarch64 darwin leading to runtime segmentation faults due to an incorrect ABI (:ghc-ticket:`21964`)
+
 Runtime system
 --------------
 

includes/CodeGen.Platform.hs

@@ -1028,6 +1028,14 @@ freeReg 29 = False
 -- ip0 -- used for spill offset computations
 freeReg 16 = False
 
+#if defined(darwin_HOST_OS) || defined(ios_HOST_OS)
+-- x18 is reserved by the platform on Darwin/iOS, and can not be used
+-- More about ARM64 ABI that Apple platforms support:
+-- https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms
+-- https://github.com/Siguza/ios-resources/blob/master/bits/arm64.md
+freeReg 18 = False
+#endif
+
 # if defined(REG_Base)
 freeReg REG_Base  = False
 # endif