Benchmarks: Set deployment target to Swift 5.5-era Apple OSes

We can probably bump this further, but at the very least this allows
the use of basic concurrency features.

Author: slavapestov

benchmark/Package.swift

@@ -185,10 +185,15 @@ targets += multiSourceLibraries.map { lib in
 // Top Level Definition
 //
 
-let p = Package(
+var p = Package(
   name: "swiftbench",
   products: products,
   targets: targets,
   swiftLanguageVersions: [.v4],
   cxxLanguageStandard: .cxx20
 )
+
+// Let's build for Swift 5.5-aligned runtimes.
+#if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+p.platforms = [.macOS(.v12), .iOS(.v15), .watchOS(.v8), .tvOS(.v15)]
+#endif