Dynamically look up backtrace(3) on Android.

The `backtrace()` function on Android was added with API level 33, but at least
some external Android builds target earlier Android NDKs, so we'll dynamically
look up the function. If it's not present or dynamic loading isn't available, we
just produce an empty backtrace.

Resolves #1135.

Author: grynspan

Sources/Testing/SourceAttribution/Backtrace.swift

@@ -40,6 +40,16 @@ public struct Backtrace: Sendable {
     self.addresses = addresses.map { Address(UInt(bitPattern: $0)) }
   }
 
+#if os(Android) && !SWT_NO_DYNAMIC_LINKING
+  /// The `backtrace()` function.
+  ///
+  /// This function was added to Android with API level 33, which is higher than
+  /// our minimum deployment target, so we look it up dynamically at runtime.
+  private static let _backtrace = symbol(named: "backtrace").map {
+    castCFunction(at: $0, to: (@convention(c) (UnsafeMutablePointer<UnsafeMutableRawPointer?>, CInt) -> CInt).self)
+  }
+#endif
+
   /// Get the current backtrace.
   ///
   /// - Parameters:
@@ -66,9 +76,9 @@ public struct Backtrace: Sendable {
         initializedCount = .init(clamping: backtrace(addresses.baseAddress!, .init(clamping: addresses.count)))
       }
 #elseif os(Android)
-      initializedCount = addresses.withMemoryRebound(to: UnsafeMutableRawPointer.self) { addresses in
-        .init(clamping: backtrace(addresses.baseAddress!, .init(clamping: addresses.count)))
-      }
+#if !SWT_NO_DYNAMIC_LINKING
+      initializedCount = .init(clamping: _backtrace?(addresses.baseAddress!, .init(clamping: addresses.count)))
+#endif
 #elseif os(Linux) || os(FreeBSD) || os(OpenBSD)
       initializedCount = .init(clamping: backtrace(addresses.baseAddress!, .init(clamping: addresses.count)))
 #elseif os(Windows)