murdock: add selected cortex-m boards to LLVM build

.murdock

 #!/bin/sh
 
 : ${TEST_BOARDS_AVAILABLE:="samr21-xpro"}
-: ${TEST_BOARDS_LLVM_COMPILE:="native"}
+: ${TEST_BOARDS_LLVM_COMPILE:="iotlab-m3 native nrf52dk mulle nucleo-f401re samr21-xpro slstk3402a"}
 
 export RIOT_CI_BUILD=1
 export STATIC_TESTS=${STATIC_TESTS:-1}