Make example projects build with GNU/ARM toolchain

* Samples modified: ADC3_DMA, ADC_Interleaved_DMAmode2,
  DAC_SignalsGeneration, DMA_FLASH_RAM, IO_Toggle
* Added Makefile to work with GNU/ARM toolchain, needed to add source
  files from the library area.
* Copied linker file from TrueStudio project (had to add an _exit symbol)
* Added a couple more binary types to .gitignore, as well as GTAGS files