Update metadata for cygwin target

Author: Berrysoft

compiler/rustc_target/src/spec/targets/x86_64_pc_cygwin.rs

@@ -18,7 +18,7 @@ pub(crate) fn target() -> Target {
             description: Some("64-bit x86 Cygwin".into()),
             tier: Some(3),
             host_tools: Some(false),
-            std: None,
+            std: Some(true),
         },
     }
 }

src/doc/rustc/src/platform-support.md

@@ -407,7 +407,7 @@ target | std | host | notes
 [`wasm64-unknown-unknown`](platform-support/wasm64-unknown-unknown.md) | ? |  | WebAssembly
 [`x86_64-apple-tvos`](platform-support/apple-tvos.md) | ✓ |  | x86 64-bit tvOS
 [`x86_64-apple-watchos-sim`](platform-support/apple-watchos.md) | ✓ |  | x86 64-bit Apple WatchOS simulator
-[`x86_64-pc-cygwin`](platform-support/x86_64-pc-cygwin.md) | ? |  | 64-bit x86 Cygwin |
+[`x86_64-pc-cygwin`](platform-support/x86_64-pc-cygwin.md) | ✓ |  | 64-bit x86 Cygwin |
 [`x86_64-pc-nto-qnx710`](platform-support/nto-qnx.md) | ✓ |  | x86 64-bit QNX Neutrino 7.1 RTOS with default network stack (io-pkt) |
 [`x86_64-pc-nto-qnx710_iosock`](platform-support/nto-qnx.md) | ✓ |  | x86 64-bit QNX Neutrino 7.1 RTOS with new network stack (io-sock) |
 [`x86_64-pc-nto-qnx800`](platform-support/nto-qnx.md) | ✓ |  | x86 64-bit QNX Neutrino 8.0 RTOS |