RISC-V: Fix uninit riscv_subset_list::m_allow_adding_dup issue

RISC-V: Fix uninit riscv_subset_list::m_allow_adding_dup issue

We forgot to initialize m_allow_adding_dup in the constructor of
riscv_subset_list, then that will be a random value...that will lead
to a random behavior of the -march may accpet duplicate extension.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc
	(riscv_subset_list::riscv_subset_list): Init m_allow_adding_dup.
Reviewed-by: Christoph Müllner <christoph.muellner@vrull.eu>