Decrease nrf51 semaphore test stack size

Decrease the stack size of the threads used on semaphore test for
the nrf51. This fixes a crash due to being out of heap memory.

Author: c1728p9

libraries/tests/rtos/mbed/semaphore/main.cpp

@@ -40,7 +40,7 @@
 #elif (defined(TARGET_EFM32GG_STK3700)) && !defined(TOOLCHAIN_ARM_MICRO)
     #define STACK_SIZE 1536
 #elif defined(TARGET_MCU_NRF51822)
-    #define STACK_SIZE 768
+    #define STACK_SIZE 512
 #else
     #define STACK_SIZE DEFAULT_STACK_SIZE
 #endif