Mark SE-0387 as implemented in 6.1 (#2594)

With swiftlang/swift-package-manager#8051 merged, the proposal is fully implemented.

Author: MaxDesiatov

proposals/0387-cross-compilation-destinations.md

@@ -3,7 +3,7 @@
 * Proposal: [SE-0387](0387-cross-compilation-destinations.md)
 * Authors: [Max Desiatov](https://github.com/MaxDesiatov), [Saleem Abdulrasool](https://github.com/compnerd), [Evan Wilde](https://github.com/etcwilde)
 * Review Manager: [Mishal Shah](https://github.com/shahmishal)
-* Status: **Accepted**
+* Status: **Implemented (Swift 6.1)**
 * Implementation: [apple/swift-package-manager#5911](https://github.com/apple/swift-package-manager/pull/5911),
   [apple/swift-package-manager#5922](https://github.com/apple/swift-package-manager/pull/5922),
   [apple/swift-package-manager#6023](https://github.com/apple/swift-package-manager/pull/6023),