target/riscv: Add svukte extension capability variable

target/riscv: Add svukte extension capability variable

Refer to the draft of svukte extension from:
https://github.com/riscv/riscv-isa-manual/pull/1564

Svukte provides a means to make user-mode accesses to supervisor memory
raise page faults in constant time, mitigating attacks that attempt to
discover the supervisor software's address-space layout.

Signed-off-by: Fea.Wang <fea.wang@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241203034932.25185-2-fea.wang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>