Check __riscv_xlen == 64 rather than _LP64 and define _LP64 if missing. Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Brooks Davis <brooks.davis@sri.com> Closes #14128
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