freertos-smp: Update prvYieldCore() to fix compile warning
freertos-smp: Update prvYieldCore() to fix compile warning

prvYieldCore() leads to an array-out-of-bounds error when compiled with
-Os optimization and configNUM_CORES = 1. This commit avoids this
compile warning by compiling out the part of code which is unnecessary
when configNUM_CORES is 1.

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf