Deprecate dcsr.halt

The debug spec 1.0.0-rc3 deprecates the dcsr.halt and lets the bit
become dcsr.nmip.

This commit separates the halt variable from the dcsr.nmip and
designates it as an internal variable for halt_on_reset (-H).
Additionally, this commit removes the notifying comment about the
deprecated dcsr.halt in the main loop.

Author: YenHaoChen

riscv/csrs.cc

@@ -1354,7 +1354,6 @@ bool dcsr_csr_t::unlogged_write(const reg_t val) noexcept {
   ebreaku = proc->extension_enabled('U') ? get_field(val, DCSR_EBREAKU) : false;
   ebreakvs = proc->extension_enabled('H') ? get_field(val, CSR_DCSR_EBREAKVS) : false;
   ebreakvu = proc->extension_enabled('H') ? get_field(val, CSR_DCSR_EBREAKVU) : false;
-  halt = get_field(val, DCSR_NMIP);
   v = proc->extension_enabled('H') ? get_field(val, CSR_DCSR_V) : false;
   pelp = proc->extension_enabled(EXT_ZICFILP) ?
          static_cast<elp_t>(get_field(val, DCSR_PELP)) : elp_t::NO_LP_EXPECTED;

riscv/execute.cc

@@ -216,7 +216,7 @@ void processor_t::step(size_t n)
       enter_debug_mode(DCSR_CAUSE_DEBUGINT);
     } else if (halt_request == HR_GROUP) {
       enter_debug_mode(DCSR_CAUSE_GROUP);
-    } // !!!The halt bit in DCSR is deprecated.
+    }
     else if (state.dcsr->halt) {
       enter_debug_mode(DCSR_CAUSE_HALT);
     }