Implement _copyRegularFile for WASI without sendfile (#787)

WASI doesn't have `sendfile`, so we need to implement the copy in user
space with `read` and `write`. It's not as efficient as `sendfile`, but
it's the best we can do.

(cherry picked from commit 2a6afeb)

Author: kateinoigakukun

Sources/FoundationEssentials/FileManager/FileOperations.swift

@@ -873,12 +873,31 @@ enum _FileOperations {
         let chunkSize: Int = Int(fileInfo.st_blksize)
         var current: off_t = 0
 
+        #if os(WASI)
+        // WASI doesn't have sendfile, so we need to do it in user space with read/write
+        try withUnsafeTemporaryAllocation(of: UInt8.self, capacity: chunkSize) { buffer in
+            while current < total {
+                let readSize = Swift.min(total - Int(current), chunkSize)
+                let bytesRead = read(srcfd, buffer.baseAddress, readSize)
+                guard bytesRead >= 0 else {
+                    try delegate.throwIfNecessary(errno, String(cString: srcPtr), String(cString: dstPtr))
+                    return
+                }
+                guard write(dstfd, buffer.baseAddress, bytesRead) == bytesRead else {
+                    try delegate.throwIfNecessary(errno, String(cString: srcPtr), String(cString: dstPtr))
+                    return
+                }
+                current += off_t(bytesRead)
+            }
+        }
+        #else
         while current < total {
             guard sendfile(dstfd, srcfd, &current, Swift.min(total - Int(current), chunkSize)) != -1 else {
                 try delegate.throwIfNecessary(errno, String(cString: srcPtr), String(cString: dstPtr))
                 return
             }
         }
+        #endif
     }
     #endif