Handle different ThreadDestructor signatures between Android NDK 27 and NDK 28 (#3249)

This change enables swift-nio to be compiled for Android against both
NDK 27 and NDK 28

### Motivation:

Android's NDK had been adopting [nullability
annotations](https://android.googlesource.com/platform/developers/docs/+/refs/heads/master/api-guidelines/ndk.md#nullability)
in its C headers, and they have been changing them from release to
release. The churn has mostly died down since the release of the NDK 27
(LTS), but one new change in NDK 28 affects the destructor argument for
`pthread_create`, making the existing `ThreadDestructor` typealias
invalid and leading to the following compile error when the [Android SDK
bundle](https://github.com/skiptools/swift-android-toolchain/releases)
is installed and linked to NDK 28:

```
$ swiftly run swift build --swift-sdk aarch64-unknown-linux-android28 +6.2-snapshot-2025-05-15-a

Building for debugging...
/opt/src/github/swift-everywhere/swift-nio/Sources/NIOPosix/ThreadPosix.swift:82:9: error: cannot convert value of type '@convention(c) (UnsafeMutableRawPointer?) -> UnsafeMutableRawPointer?' to expected argument type '@convention(c) (UnsafeMutableRawPointer) -> UnsafeMutableRawPointer'
 80 |         &handleLinux,
 81 |         nil,
 82 |         destructor,
    |         `- error: cannot convert value of type '@convention(c) (UnsafeMutableRawPointer?) -> UnsafeMutableRawPointer?' to expected argument type '@convention(c) (UnsafeMutableRawPointer) -> UnsafeMutableRawPointer'
 83 |         args
 84 |     )
[4/21] Compiling NIOPosix ThreadPosix.swift
```

This change adapts the parameter we pass to that function to accommodate
both NDK 27 and NDK 28.

### Modifications:

There's currently no way to determine which version of the NDK is in use
at compile time (see swiftlang/swift#81402),
so this accommodation is implementing by having two functions named
`coerceThreadDestructor` that return values that are suitable either NDK
27 or NDK 28 (using `unsafeBitCast`, but the layout of C functions
doesn't know anything about nullability, so this is safe), and then let
the type inference for `pthread_create` handle the rest.

### Result:

swift-nio can be compiled against both NDK 27 and NDK 28 without errors.

Author: marcprux

Sources/NIOPosix/ThreadPosix.swift

@@ -60,10 +60,30 @@ private func sysPthread_create(
     )
     #else
     var handleLinux = pthread_t()
+    #if os(Android)
+    // NDK 27 signature:
+    // int pthread_create(pthread_t* _Nonnull __pthread_ptr, pthread_attr_t const* _Nullable __attr, void* _Nonnull (* _Nonnull __start_routine)(void* _Nonnull), void* _Nullable);
+    func coerceThreadDestructor(
+        _ destructor: @escaping ThreadDestructor
+    ) -> (@convention(c) (UnsafeMutableRawPointer) -> UnsafeMutableRawPointer) {
+        destructor
+    }
+
+    // NDK 28 signature:
+    // int pthread_create(pthread_t* _Nonnull __pthread_ptr, pthread_attr_t const* _Nullable __attr, void* _Nullable (* _Nonnull __start_routine)(void* _Nullable), void* _Nullable);
+    func coerceThreadDestructor(
+        _ destructor: @escaping ThreadDestructor
+    ) -> (@convention(c) (UnsafeMutableRawPointer?) -> UnsafeMutableRawPointer?) {
+        unsafeBitCast(destructor, to: (@convention(c) (UnsafeMutableRawPointer?) -> UnsafeMutableRawPointer?).self)
+    }
+    #else
+    // Linux doesn't change the signature
+    func coerceThreadDestructor(_ destructor: ThreadDestructor) -> ThreadDestructor { destructor }
+    #endif
     let result = pthread_create(
         &handleLinux,
         nil,
-        destructor,
+        coerceThreadDestructor(destructor),
         args
     )
     #endif