Disable Intel MPX by default

The next generation of Intel CPUs does not support Intel MPX. Enabling MPX
by default crashes the LibOS on startup. So we disable MPX by default. The
long term plan is to turn on/off MPX via compiling options.

src/libos/src/entry.rs

@@ -43,7 +43,7 @@ pub extern "C" fn occlum_ecall_init(log_level: *const c_char) -> i32 {
         // Init the log infrastructure first so that log messages will be printed afterwards
         util::log::init(log_level);
         // Init MPX for SFI
-        util::mpx_util::mpx_enable();
+        //util::mpx_util::mpx_enable();
         // Register exception handlers (support cpuid & rdtsc for now)
         register_exception_handlers();