Wrap software_init_hook

Wrap software_init_hook so that it can be used or extended from outside the
RTOS. This is desirable so that code can be added to the software_init_hook
without making the RTOS depend on new features or libraries.

hal/common/retarget.cpp

@@ -412,6 +412,18 @@ extern "C" WEAK void __cxa_pure_virtual(void) {
 
 #endif
 
+#if defined(TOOLCHAIN_GCC)
+extern "C" WEAK void software_init_hook_rtos(void)
+{
+    // Do nothing by default.
+}
+
+extern "C" void software_init_hook(void)
+{
+    software_init_hook_rtos();
+}
+#endif
+
 // ****************************************************************************
 // mbed_main is a function that is called before main()
 // mbed_sdk_init() is also a function that is called before main(), but unlike

rtos/rtx/TARGET_ARM7/RTX_CM_lib.h

@@ -339,7 +339,7 @@ void pre_main(void) {
     main(0, NULL);
 }
 
-__attribute__((naked)) void software_init_hook (void) {
+__attribute__((naked)) void software_init_hook_rtos (void) {
   __asm (
     ".syntax unified\n"
     ".thumb\n"

rtos/rtx/TARGET_CORTEX_A/RTX_CM_lib.h

@@ -447,7 +447,7 @@ __asm void __rt_entry (void) {
 
 #elif defined (__GNUC__)
 
-__attribute__((naked)) void software_init_hook (void) {
+__attribute__((naked)) void software_init_hook_rtos (void) {
   __asm (
     ".syntax unified\n"
     ".arm\n"

rtos/rtx/TARGET_CORTEX_M/RTX_CM_lib.h

@@ -655,7 +655,7 @@ void pre_main(void) {
     main(0, NULL);
 }
 
-__attribute__((naked)) void software_init_hook (void) {
+__attribute__((naked)) void software_init_hook_rtos (void) {
   __asm (
     "bl   osKernelInitialize\n"
 #ifdef __MBED_CMSIS_RTOS_CM