Synchronization: Add errno to some linux lock fatalErrors

In the comments it's stated that these errnos can't really occur for
the implementation, but unfortunately they have been seen in some
scenarios. To aide in debugging, it'd be nice to at least have the
errno in the error message.

Author: dcantah

stdlib/public/Synchronization/Mutex/LinuxImpl.swift

@@ -137,7 +137,8 @@ extension _MutexHandle {
       // Block until an equivalent '_futexUnlock' has been called by the owner.
       // This returns '0' on success which means the kernel has acquired the
       // lock for us.
-      switch storage._futexLock() {
+      let lockResult = storage._futexLock()
+      switch lockResult {
       case 0:
         // Locked!
         return
@@ -191,7 +192,7 @@ extension _MutexHandle {
       // ESRCH  - "The thread ID in the futex word at uaddr does not exist."
       default:
         // TODO: Replace with a colder function / one that takes a StaticString
-        fatalError("Unknown error occurred while attempting to acquire a Mutex")
+        fatalError("Unknown error occurred while attempting to acquire a Mutex: \(lockResult)")
       }
     }
   }
@@ -308,7 +309,8 @@ extension _MutexHandle {
   @usableFromInline
   internal borrowing func _unlockSlow() {
     while true {
-      switch storage._futexUnlock() {
+      let unlockResult = storage._futexUnlock()
+      switch unlockResult {
       case 0:
         // Unlocked!
         return
@@ -360,7 +362,7 @@ extension _MutexHandle {
       //           space.)"
       default:
         // TODO: Replace with a colder function / one that takes a StaticString
-        fatalError("Unknown error occurred while attempting to release a Mutex")
+        fatalError("Unknown error occurred while attempting to release a Mutex: \(unlockResult)")
       }
     }
   }