linux-user/riscv: Propagate fault address

linux-user/riscv: Propagate fault address

The CPU loop tagged all the queued signals as QEMU_SI_KILL while it was
filling the `_sigfault` part of `siginfo`: this caused QEMU to copy the
wrong fields over to the userspace program.

Make sure the fault address recorded by the MMU is is stored in the CPU
environment structure.

In case of memory faults store the exception address into `siginfo`.

Signed-off-by: Giuseppe Musacchio <[email protected]>
Reviewed-by: Palmer Dabbelt <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>