kernel: implement NX (No-Execute) support via IA32_EFER.NXE

kernel: implement NX (No-Execute) support via IA32_EFER.NXE

Fix A01 (W^X/NX) which was deferred due to IA32_EFER.NXE MSR instability.
Root cause: NX bit was being set in PTEs without NXE enabled, causing
undefined behavior and kernel panic.

Changes:
- boot.S: Check CPUID.0x80000001:EDX bit 20 for NX support before enabling
- boot.S: Enable IA32_EFER.NXE (MSR 0xC0000080, bit 11) if NX supported
- vmm.c: Add g_nxe_enabled flag and check_nxe_enabled() function
- vmm.c: Conditionalize X86_PTE_NX usage based on g_nxe_enabled
- vmm.c: Print NX status in vmm_init()
- Makefile: Add -cpu qemu32,+nx to expose NX support in QEMU
- smoke_test.exp: Add -cpu qemu32,+nx for testing

Behavior:
- With NX support: NXE enabled, VMM uses NX bit for non-executable pages
- Without NX support: NXE not enabled, VMM ignores VMM_FLAG_NX
- W^X now works correctly for ELF loading, mmap/mprotect, etc.

Test: 119/119 PASS (SMP=4)