Zicond: implement Zicond (conditional integer operations)

This implements the Zicond (conditional integer operations) extension,
as of version 1.0-draft-20230120.

The Zicond extension acts as a building block for branchless sequences
including conditional-arithmetic, conditional-logic and
conditional-select/move.
The following instructions constitute Zicond:
 - czero.eqz rd, rs1, rs2  =>  rd = (rs2 == 0) ? 0 : rs1
 - czero.nez rd, rs1, rs2  =>  rd = (rs2 != 0) ? 0 : rs1

See
  https://github.com/riscv/riscv-zicond/releases/download/v1.0-draft-20230120/riscv-zicond_1.0-draft-20230120.pdf
for the proposed specification and usage details.

riscv/insns/czero_eqz.h

@@ -0,0 +1,2 @@
+require_extension(EXT_ZICOND);
+WRITE_RD(RS2 == 0 ? 0 : RS1);

riscv/insns/czero_nez.h

@@ -0,0 +1,2 @@
+require_extension(EXT_ZICOND);
+WRITE_RD(RS2 != 0 ? 0 : RS1);

riscv/isa_parser.cc

@@ -220,6 +220,8 @@ isa_parser_t::isa_parser_t(const char* str, const char *priv)
       extension_table[EXT_ZICBOZ] = true;
     } else if (ext_str == "zicbop") {
     } else if (ext_str == "zicntr") {
+    } else if (ext_str == "zicond") {
+      extension_table[EXT_ZICOND] = true;
     } else if (ext_str == "zihpm") {
     } else if (ext_str == "sstc") {
         extension_table[EXT_SSTC] = true;

riscv/isa_parser.h

@@ -53,6 +53,7 @@ typedef enum {
   EXT_ZICBOM,
   EXT_ZICBOZ,
   EXT_ZICNTR,
+  EXT_ZICOND,
   EXT_ZIHPM,
   EXT_XZBP,
   EXT_XZBS,

riscv/riscv.mk.in

@@ -1305,6 +1305,10 @@ riscv_insn_ext_cmo = \
 	cbo_inval \
 	cbo_zero \
 
+riscv_insn_ext_zicond = \
+        czero_eqz \
+        czero_nez \
+
 riscv_insn_list = \
 	$(riscv_insn_ext_a) \
 	$(riscv_insn_ext_c) \
@@ -1323,6 +1327,7 @@ riscv_insn_list = \
 	$(riscv_insn_priv) \
 	$(riscv_insn_svinval) \
 	$(riscv_insn_ext_cmo) \
+	$(riscv_insn_ext_zicond) \
 
 riscv_gen_srcs = $(addsuffix .cc,$(riscv_insn_list))