The “Convert to C++” feature is not fully complete yet. The linker options are not persistent between C and C++.
You have to manually change the following values in Properties > C/C++ Build > Settings > MCU G++ Linker :
In General > Linker Script, change the ld script into the one in your project root directory (the name pattern should be ../STM32FXXXXx_FLASH.ld)
In Miscellanous > Linker Flags, add -specs=nosys.specs -specs=nano.specs
The problem has been reported and these options will be automically changed in the next release.
Hope it works