Added core description file for ARM Cortex-M4.

This needs changes but for now it is OK.
Added a basic file and config set to start porting for ARM Cortex-M4 devices.
This adds only some linker file from the STM firmware examples that links everything in the internal flash.
I hardcoded the usage of the code sourcery arm compiler if the GCCPREFIX macro is not already defined.

No further changes were made to the files. They are just a plain copy of sys/pic32/baremetal as mentioned in the following blog post:
http://retrobsd.org/viewtopic.php?f=3&t=2545#p2568

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I think this is a good starting point to think about a proper memory map and write a corresponding linker file.
Added new target: STM32
Now we are able to compile and link against the setup code sourcery arm baremetal compiler.
All files within sys/stm32/* are taken into account (if defined in the *.dev files or cfg files).