[M487] Remove mbed_sdk_init_forced()

1. With mbed OS 5, mbed_sdk_init() is ensured to call before C++ global object constructor.
2. Refine startup file with GCC_ARM toolchain related to this modification.

targets/TARGET_NUVOTON/TARGET_M480/TARGET_NUMAKER_PFM_M487/mbed_overrides.c

@@ -16,14 +16,6 @@
 
 #include "analogin_api.h"
 
-// NOTE: Ensure mbed_sdk_init() will get called before C++ global object constructor.
-#if defined(__CC_ARM) || defined(__GNUC__)
-void mbed_sdk_init_forced(void) __attribute__((constructor(101)));
-#elif defined(__ICCARM__)
-    // FIXME: How to achieve it in IAR?
-#endif
-
-
 void mbed_sdk_init(void)
 {
     // NOTE: Support singleton semantics to be called from other init functions
@@ -78,8 +70,3 @@ void mbed_sdk_init(void)
     /* Lock protected registers */
     SYS_LockReg();
 }
-
-void mbed_sdk_init_forced(void)
-{
-    mbed_sdk_init();
-}

targets/TARGET_NUVOTON/TARGET_M480/device/startup_M480.c

@@ -62,12 +62,11 @@ extern uint32_t __bss_start__;
 extern uint32_t __bss_end__;
 
 extern void uvisor_init(void);
-//#if defined(TOOLCHAIN_GCC_ARM)
-//extern void _start(void);
-//#endif
-extern void software_init_hook(void) __attribute__((weak));
-extern void __libc_init_array(void);
-extern int main(void);
+#if defined(TOOLCHAIN_GCC_ARM)
+extern void _start(void);
+#else
+#error("For GCC toolchain, only support GNU ARM Embedded")
+#endif
 #endif
 
 /* Default empty handler */
@@ -463,17 +462,8 @@ void Reset_Handler_2(void)
         }
     }
     
-    if (software_init_hook) {
-        /**
-         * Give control to the RTOS via software_init_hook() which will also call __libc_init_array().
-         * Assume software_init_hook() is defined in libraries/rtos/rtx/TARGET_CORTEX_M/RTX_CM_lib.h.
-         */
-        software_init_hook();
-    }
-    else {
-        __libc_init_array();
-        main();
-    }
+    _start();
+    
 #endif
 
     /* Infinite loop */