GDB overlay support for RISC-V

This is custom build of GDB-client with overlay support for RISC-V and TUI.
It was built with using project sifive/freedom-gdb-metal.

Done:

1. Fixed problem with overlay support for RISC-V. Now GDB-client supports overlay debugging in auto-mode.
2. Fixed problem with output of overlay GDB-commands (overlay auto, overlay manual, overlay off).

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My overlay demo project for RISC-V: dmi391/overlay_demo