The full multiply intrinsics are not included for EEW=64 in Zve64*.
They require the V extension to be enabled.
This commit improves diagnostic message from
<source>:4:10: error: call to undeclared function '__riscv_vsmul_vv_i64m1'; 4 | return __riscv_vsmul_vv_i64m1(op1, op2, __RISCV_VXRM_RNU, vl);
to
test.c:5:10: error: builtin requires: v 5 | return __riscv_vsmul_vv_i64m1(op1, op2, __RISCV_VXRM_RNU, vl);