RISC-V: Fix one build error for template default arg

RISC-V: Fix one build error for template default arg

In some build option combination, the default value may result in
below error. This patch would like to fix it by passing a explict
argument.

riscv-vector-builtins-bases.cc:2495:24: error: invalid use of template-name \
  ‘riscv_vector::vfcvt_f’ without an argument list

Signed-off-by: Pan Li <pan2.li@intel.com>
gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-bases.cc: Use explicit argument.