Fix Android builds with NDK r28 and later

The nullability annotations of many POSIX functions changed in r28. Also switch the CI to test with both the latest LTS version (r27d) and the latest stable version (r29).

Author: jakepetroules

.github/workflows/pull_request.yml

@@ -39,6 +39,7 @@ jobs:
       macos_build_command: 'xcrun swift-format lint -s -r --configuration ./.swift-format . && xcrun swift test && xcrun swift test -c release && xcrun swift test --disable-default-traits'
       enable_linux_static_sdk_build: true
       enable_android_sdk_build: true
+      android_ndk_version: '["r27d", "r29"]'
       linux_static_sdk_versions: '["6.1", "nightly-6.2"]'
       linux_static_sdk_build_command: |
         for triple in aarch64-swift-linux-musl x86_64-swift-linux-musl ; do

Sources/Subprocess/Thread.swift

@@ -318,7 +318,7 @@ internal func pthread_create(
         return context
     }
     #elseif canImport(Bionic)
-    func proc(_ context: UnsafeMutableRawPointer) -> UnsafeMutableRawPointer {
+    func proc(_ context: UnsafeMutableRawPointer!) -> UnsafeMutableRawPointer! {
         (Unmanaged<AnyObject>.fromOpaque(context).takeRetainedValue() as! Context).body()
         return context
     }