riscv: add hardware error trap handler support

riscv: add hardware error trap handler support

Add support for handling hardware error traps (exception code 19)
in the RISC-V architecture. The changes include:

- Add do_trap_hardware_error function declaration in asm-prototypes.h
- Add hardware error trap vector entry in entry.S exception vector table
- Implement do_trap_hardware_error handler in traps.c that generates
  SIGBUS with BUS_MCEERR_AR for hardware errors

This enables proper handling of hardware error exceptions that may occur
in RISC-V systems, providing appropriate error reporting and signal
generation for user space processes.

Signed-off-by: Rui Qi <qirui.001@bytedance.com>
Link: https://patch.msgid.link/20260202094200.53735-1-qirui.001@bytedance.com
[pjw@kernel.org: clean up commit message slightly]
Signed-off-by: Paul Walmsley <pjw@kernel.org>