builtin.closure: use volatile register for s390x (fix #24383) (#26256)

Author: tankf33der

vlib/builtin/closure/closure.c.v

@@ -69,10 +69,10 @@ pub const closure_thunk = $if amd64 {
     ]!
 } $else $if s390x {
     [
-    u8(0xC0), 0x70, 0xFF, 0xFF, 0xE0, 0x00,  // larl %r7, -16384
-        0x68, 0xF0, 0x70, 0x00,              // ld   %f15, 0(%r7)
-        0xE3, 0x70, 0x70, 0x08, 0x00, 0x04,  // lg   %r7, 8(%r7)
-        0x07, 0xF7,                          // br   %r7
+    u8(0xC0), 0x10, 0xFF, 0xFF, 0xE0, 0x00,  // larl %r1, -16384
+        0x68, 0xF0, 0x10, 0x00,              // ld   %f15, 0(%r1)
+        0xE3, 0x10, 0x10, 0x08, 0x00, 0x04,  // lg   %r1, 8(%r1)
+        0x07, 0xF1,                          // br   %r1
     ]!
 } $else $if ppc64le {
     [