target/riscv: Add svukte extension capability variable

Refer to the draft of svukte extension from:
riscv/riscv-isa-manual#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>

target/riscv/cpu_cfg.h

@@ -84,6 +84,7 @@ struct RISCVCPUConfig {
     bool ext_svnapot;
     bool ext_svpbmt;
     bool ext_svvptc;
+    bool ext_svukte;
     bool ext_zdinx;
     bool ext_zaamo;
     bool ext_zacas;