fix: add randUint64 wrapper for Go 1.21 compatibility

Implement randUint64() directly in the maphash overlay to ensure compatibility
across Go versions. Go 1.21's randUint64() calls runtime.fastrand64, while
Go 1.22+ calls runtime.rand. The wrapper function bridges to llgo's fastrand64
implementation, avoiding undefined symbol errors on Go 1.21.

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Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>

runtime/internal/lib/hash/maphash/maphash.go

@@ -23,3 +23,7 @@ func runtime_rand() uint64
 
 //go:linkname runtime_memhash github.com/goplus/llgo/runtime/internal/runtime.memhash
 func runtime_memhash(p unsafe.Pointer, seed, s uintptr) uintptr
+
+func randUint64() uint64 {
+	return runtime_rand()
+}