windows: fix wrong register for first parameter

I'm surprised this worked as long as it did, since it looks like the
goroutine stack did not get scanned. Or maybe the RCX register contained
the stack pointer by accident. In any case, it now uses the correct
register (RCX instead of RDI on Windows) for passing the stack pointer
as the first parameter.

Author: aykevl

src/runtime/asm_amd64_windows.S

@@ -12,8 +12,8 @@ tinygo_scanCurrentStack:
     pushq %r15
 
     // Scan the stack.
-    subq $8, %rsp // adjust the stack before the call to maintain 16-byte alignment
-    movq %rsp, %rdi
+    subq $8, %rsp   // adjust the stack before the call to maintain 16-byte alignment
+    movq %rsp, %rcx // pass the stack pointer as the first parameter
     callq tinygo_scanstack
 
     // Restore the stack pointer. Registers do not need to be restored as they