Dynamically look up backtrace(3) on Android. (swiftlang#1356)

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.

See:
https://cs.android.com/android/_/android/platform/bionic/+/731631f300090436d7f5df80d50b6275c8c60a93:libc/include/execinfo.h;l=52

Resolves swiftlang#1135.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

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)