target/riscv: Add M-mode virtual interrupt and IRQ filtering support.

target/riscv: Add M-mode virtual interrupt and IRQ filtering support.

This change adds support for inserting virtual interrupts from M-mode
into S-mode using mvien and mvip csrs. IRQ filtering is a use case of
this change, i-e M-mode can stop delegating an interrupt to S-mode and
instead enable it in MIE and receive those interrupts in M-mode and then
selectively inject the interrupt using mvien and mvip.

Also, the spec doesn't mandate the interrupt to be actually supported
in hardware. Which allows M-mode to assert virtual interrupts to S-mode
that have no connection to any real interrupt events.

This is defined as part of the AIA specification [0], "5.3 Interrupt
filtering and virtual interrupts for supervisor level".

[0]: https://github.com/riscv/riscv-aia/releases/download/1.0/riscv-interrupts-1.0.pdf

Signed-off-by: Rajnesh Kanwal <rkanwal@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20231016111736.28721-6-rkanwal@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>