Fix Clang error when disabling BLOCK_CHAINING

When using 'make ENABLE_MBLOCK_CHAINING=0 CC=clang', the following
errors are observed:
src/emulate.c:555:19: error: unused function 'insn_is_unconditional_branch' [-Werror,-Wunused-function]
  555 | FORCE_INLINE bool insn_is_unconditional_branch(uint8_t opcode)
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/emulate.c:582:19: error: unused function 'insn_is_direct_branch' [-Werror,-Wunused-function]
  582 | FORCE_INLINE bool insn_is_direct_branch(uint8_t opcode)
      |                   ^~~~~~~~~~~~~~~~~~~~~

Fix this by guarding the 'insn_is_unconditional_branch' and
'insn_is_direct_branch' function definition with
RV32_HAS(BLOCK_CHAINING) macro. This ensures the functions are only
defined when the BLOCK_CHAINING is enabled.

Author: eleanorLYJ

src/emulate.c

@@ -552,6 +552,7 @@ FORCE_INLINE bool insn_is_translatable(uint8_t opcode)
 }
 #endif
 
+#if RV32_HAS(BLOCK_CHAINING)
 FORCE_INLINE bool insn_is_unconditional_branch(uint8_t opcode)
 {
     switch (opcode) {
@@ -592,6 +593,7 @@ FORCE_INLINE bool insn_is_direct_branch(uint8_t opcode)
         return false;
     }
 }
+#endif
 
 FORCE_INLINE bool insn_is_indirect_branch(uint8_t opcode)
 {