Add ability to run custom programs in tb/core

Created by: bluewww

There is an example on how to do this in tb/core/custom on how such a program could look like and the new makefile sections shows how to compile and link it.

• Extend the testbench to handle exit codes
• Add minimal syscall implementation for newlib
• Add example on how to link against newlib and custom crt0.s in makefile
• Add example linker script showing how to put stack, heap, .text, .data, .bss etc. into an elf
• Add example on how to handle interrupts
• Clean up makefile targets for hex generation