Use correct format string for $value$plusargs

Not doing so causes VCS to spit out a warning message. The intention
seems to be that the initial call to $value$plusargs will evaluate to
true and will put the value that was assigned into the
disable_pmp_exception_handler variable, which then gets checked.

Author: rswarbrick

dv/uvm/core_ibex/riscv_dv_extension/ibex_asm_program_gen.sv

@@ -54,7 +54,7 @@ class ibex_asm_program_gen extends riscv_asm_program_gen;
 
     riscv_csr_instr::create_csr_filter(cfg);
 
-    if ($value$plusargs("disable_pmp_exception_handler", disable_pmp_exception_handler) &&
+    if ($value$plusargs("disable_pmp_exception_handler=%d", disable_pmp_exception_handler) &&
         disable_pmp_exception_handler) begin
       cfg.pmp_cfg.enable_pmp_exception_handler = 0;
     end