I recently ran into this exact same issue, and ultimately resolved it by pulling in the syscalls.c implementation from the STM32CubeF4 samples which are downloadable from the st.com site for your board. Below is the folder I found a suitable version of the implementations for my discovery board.
C:stm32cubef4STM32Cube_FW_F4_V1.3.0ProjectsSTM32F429I-DiscoveryExamplesBSPTrueSTUDIO