RISC-V: Add "m_" prefix for private member

RISC-V: Add "m_" prefix for private member

Since the current framework is hard to maintain and
hard to be used in the future possible auto-vectorization patterns.

We will need to keep adding more helpers and arguments during the
auto-vectorization supporting. We should refactor the framework
now for the future use since the we don't support too much
auto-vectorization
patterns for now.

Start with this simple patch, this patch is adding "m_" prefix for
private the members.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc: Add "m_" prefix.

Signed-off-by: Juzhe-Zhong <juzhe.zhong@rivai.ai>