riscv: support trap-based WARN()

The WARN() related function will trigger a debug exception. This can help
developers to analyze the cause of WARN() because if the debugger is
connected, the control flow will be transferred to debugging
environment.

Signed-off-by: Vincent Chen <vincentc@andestech.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>