Make Realtek link

There was some interesting stuff here.
 * ARMC6 treats assembly the same as GNU, but has a different startup
   function, so we branch the asm there
 * Incorrect discriminant, probably copied from other ports

targets/TARGET_Realtek/TARGET_AMEBA/TARGET_RTL8195A/device/rtl8195a_init.c

@@ -270,7 +270,11 @@ void PLAT_Main(void)
 #else
     __asm ("ldr  r0, =SystemInit   \n"
            "blx  r0                \n"
+#if defined(__ARMCC_VERSION) && (__ARMCC_VERSION >= 6010050)
+           "ldr  r0, =__main \n"
+#else
            "ldr  r0, =_start       \n"
+#endif
            "bx   r0                \n"
     );
 #endif

targets/TARGET_Realtek/mbed_rtx.h

@@ -20,7 +20,7 @@
 
 #include "rtl8195a.h"
 
-#if defined(__CC_ARM)
+#if defined(__CC_ARM) || (defined(__ARMCC_VERSION) || (__ARMCC_VERSION >= 6010050))
     extern uint32_t             Image$$ARM_LIB_STACK$$ZI$$Base[];
     extern uint32_t             Image$$ARM_LIB_STACK$$ZI$$Length[];
     #define ISR_STACK_START     (unsigned char *)(Image$$ARM_LIB_STACK$$ZI$$Base)