Why choose libraries based on riscv64-* or riscv32-**instead of xlen?

I found that when I use the compiler riscv64, no matter ```-march = rv32gc``` or ```-march = rv64gc```, ```typedef signed long int64_t;``` always appears. Obviously, when ```-march = rv32gc```, it is correct to ```typedef the signed long long int64_t;```.