litex_setup: fix software build when liteeth or/and litesata is set (riscv toolchain issue)

[trabucayre]

Starting with ubuntu 22.04, when a target is configured with ethernet and/or sata the sofware build fails with:

/usr/lib/gcc-cross/riscv64-linux-gnu/12/../../../../riscv64-linux-gnu/bin/ld: ../libc/libc.a(libc_ssp_chk_fail.c.o): in function `__chk_fail':
/somewhere/litex-boards/litex_boards/targets/build/digilent_arty/software/libc/../../../../../../../pythondata-software-picolibc/pythondata_software_picolibc/data/newlib/libc/ssp/chk_fail.c:13: undefined reference to `write'
collect2: error: ld returned 1 exit status
make: *** [/somewhere/litex/litex/soc/software/bios/Makefile:72: bios.elf] Error 1

By switching to gcc-riscv64-unknown-elf (bare-metal toolchain) build end successfully
Fix:

• Ubuntu 22.04 riscv gcc causes issue #1624
• Default bios not compiling for tang primer 20k, #1621

[enjoy-digital]

Thanks @trabucayre!

[jevinskie]

Just FYI if we wanted to consolidate on the same RISC-V toolchain for every host we might be able to use these prebuilds. After looking around for a bit this packaging project seems to be the most similar to other high-quality “universal” crosstool builds like Linaro’s Arm cross-compiler releases.