[android] add android build support

this is needed to build swift-corelibs-foundation for android, using the 'package' branch that is based on new swift-foundation

Sources/SwiftLibraryPluginProvider/LibraryPluginProvider.swift

@@ -22,6 +22,8 @@ private import Darwin
 private import Glibc
 #elseif canImport(Musl)
 private import Musl
+#elseif canImport(Android)
+private import Android
 #endif
 #else
 import SwiftSyntaxMacros
@@ -33,6 +35,8 @@ import SwiftSyntaxMacros
 @_implementationOnly import Glibc
 #elseif canImport(Musl)
 @_implementationOnly import Musl
+#elseif canImport(Android)
+@_implementationOnly import Android
 #endif
 #endif
 
@@ -137,7 +141,12 @@ private func _loadLibrary(_ path: String) throws -> UnsafeMutableRawPointer {
 #else
 private func _loadLibrary(_ path: String) throws -> UnsafeMutableRawPointer {
   guard let dlHandle = dlopen(path, RTLD_LAZY | RTLD_LOCAL) else {
-    throw LibraryPluginError(message: "loader error: \(String(cString: dlerror()))")
+#if canImport(Android)
+    let err = dlerror()!
+#else
+    let err = dlerror()
+#endif
+    throw LibraryPluginError(message: "loader error: \(String(cString: err))")
   }
   return dlHandle
 }