riscv: add support for backtrace

[ssumpf]

Add support for the Genode::backtrace call for RISC-V platforms when compiled with -fno-omit-frame-pointer.

[ssumpf]

The commits above fix some problems with staging and enable Genode::backtrace for RISC-V.

[nfeske]

Very nice! Thank you @ssumpf. Merged to staging.

[nfeske]

With c7126e3 the bin/riscv/base-hw-virt_qemu_riscv depot archive can be built now as well.