[stdlib] Refactoring some ifdef code

Patched for compnerd review in PR #3886.

Author: tinysun212

stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift

@@ -98,20 +98,14 @@ public func _stdlib_select(
         let writeAddr = writefds.baseAddress
         let errorAddr = errorfds.baseAddress
 #if CYGWIN
-        func asFdSetPtr(
-          _ p: UnsafeMutablePointer<UInt>?
-        ) -> UnsafeMutablePointer<_types_fd_set>? {
-          return UnsafeMutableRawPointer(p)?
-            .assumingMemoryBound(to: _types_fd_set.self)
-        }
-#else
+        typealias fd_set = _types_fd_set
+#endif
         func asFdSetPtr(
           _ p: UnsafeMutablePointer<UInt>?
         ) -> UnsafeMutablePointer<fd_set>? {
           return UnsafeMutableRawPointer(p)?
             .assumingMemoryBound(to: fd_set.self)
         }
-#endif
         return select(
           _stdlib_FD_SETSIZE,
           asFdSetPtr(readAddr),

stdlib/public/Platform/glibc.modulemap.gyb

@@ -331,12 +331,10 @@ module SwiftGlibc [system] {
       }
 % end
 
-% if CMAKE_SDK in ["CYGWIN"]:
       module file {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/file.h"
         export *
       }
-% end
 
       module ipc {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/ipc.h"