Fix unused variable warning when JIT and SYSTEM are enabled

When building with make 'ENABLE_JIT=1 ENABLE_SYSTEM=1', the following
error is observed:

src/emulate.c:45:13: error: ‘need_clear_block_map’ defined but not used [-Werror=unused-variable]
   45 | static bool need_clear_block_map = false;
      |             ^~~~~~~~~~~~~~~~~~~~

Fix this by guarding the 'need_clear_block_map' definition with the
RV32_HAS(SYSTEM) and !RV32_HAS(JIT) macros. This ensures the variable
is only defined when the macros indicate it is needed.

Author: eleanorLYJ

src/emulate.c

@@ -42,7 +42,9 @@ extern struct target_ops gdbstub_ops;
 #define IF_imm(i, v) (i->imm == v)
 
 #if RV32_HAS(SYSTEM)
+#if !RV32_HAS(JIT)
 static bool need_clear_block_map = false;
+#endif
 static uint32_t reloc_enable_mmu_jalr_addr;
 static bool reloc_enable_mmu = false;
 bool need_retranslate = false;