Huh. New issue now with your config, this time in build.
...
Compiling .pioenvs\genericSTM32F103C8\FrameworkArduino\wiring_time.c.o
Archiving .pioenvs\genericSTM32F103C8\libFrameworkArduino.a
Indexing .pioenvs\genericSTM32F103C8\libFrameworkArduino.a
Linking .pioenvs\genericSTM32F103C8\firmware.elf
c:/users/neilb/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/8.2.1/../../../../arm-none-eabi/bin/ld.exe: .pioenvs\genericSTM32F103C8\libFrameworkArduino.a(syscalls.c.o): in function `_sbrk':
syscalls.c:(.text._sbrk+0x38): undefined reference to `_Min_Stack_Size'
c:/users/neilb/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/8.2.1/../../../../arm-none-eabi/bin/ld.exe: .pioenvs\genericSTM32F103C8\libFrameworkArduino.a(syscalls.c.o):(.data.heap_end.7830+0x0): undefined reference to `_end'
collect2.exe: error: ld returned 1 exit status
*** [.pioenvs\genericSTM32F103C8\firmware.elf] Error 1