Fix single-threaded apps for IAR v8 builds

For single-threaded apps __iar_Initlocks() is not required and must
not be called because the proper IAR library is brought in only if
"--threaded_lib" is defined in the linker command.

rtos/TARGET_CORTEX/TOOLCHAIN_IAR/mbed_boot_iar.c

@@ -87,7 +87,7 @@ void __iar_program_start(void)
 
 void mbed_toolchain_init(void)
 {
-#if defined(__IAR_SYSTEMS_ICC__ ) && (__VER__ >= 8000000)
+#if defined(__IAR_SYSTEMS_ICC__ ) && (__VER__ >= 8000000) && !defined(MBED_RTOS_SINGLE_THREAD)
     __iar_Initlocks();
 #endif