Fix JIT compilation with SYSTEM/ELF_LOADER enabled

[jserv]

This introduces SYSTEM_MMIO feature flag to properly gate MMIO-related code in JIT. This flag is enabled only when SYSTEM=1 and ELF_LOADER=0 (Linux kernel mode), preventing compilation errors when both SYSTEM and ELF_LOADER are enabled.

---

Summary by cubic

Fix JIT build when SYSTEM and ELF_LOADER are both enabled by gating MMIO code to Linux kernel mode only. This prevents compilation errors while keeping MMIO for kernel emulation.

• Bug Fixes
  • Introduced RV32_FEATURE_SYSTEM_MMIO (enabled when SYSTEM=1 and ELF_LOADER=0).
  • Switched JIT MMIO checks to RV32_HAS(SYSTEM_MMIO) in jit.c and rv32_jit.c.
  • Scoped MMIO macros in system.h behind !RV32_HAS(ELF_LOADER) to avoid conflicts.

[ChinYikMing]

The compilation error can be reproduced via make ENABLE_SYSTEM=1 ENABLE_ELF_LOADER=1 ENABLE_JIT=1 -j8 without these changes.

Sample errors:

...

src/jit.c: In function ‘jit_mmio_read_wrapper’:
src/jit.c:1401:5: error: implicit declaration of function ‘MMIO_READ’ [-Werror=implicit-function-declaration]
 1401 |     MMIO_READ();
      |     ^~~~~~~~~
src/jit.c:1399:41: error: unused parameter ‘rv’ [-Werror=unused-parameter]
 1399 | uint32_t jit_mmio_read_wrapper(riscv_t *rv, uint32_t addr)
      |                                ~~~~~~~~~^~
src/jit.c:1399:54: error: unused parameter ‘addr’ [-Werror=unused-parameter]
 1399 | uint32_t jit_mmio_read_wrapper(riscv_t *rv, uint32_t addr)
      |                                             ~~~~~~~~~^~~~
src/jit.c: In function ‘jit_mmu_handler’:
src/jit.c:1430:9: error: implicit declaration of function ‘MMIO_WRITE’ [-Werror=implicit-function-declaration]
 1430 |         MMIO_WRITE();
      |         ^~~~~~~~~~
src/jit.c:1424:14: error: variable ‘val’ set but not used [-Werror=unused-but-set-variable]
 1424 |     uint32_t val;
 
 ...

With these changes applied, the compilation errors are gone.

LGTM!