add GCC triplet for xPack RISC-V Embedded GCC

[ilg-ul]

The xPack RISC-V Embedded GCC toolchain uses the riscv-none-elf triplet.

This patch adds it to the PICO_DEFAULT_GCC_TRIPLE list in the CMake files, thus adding support for the xPack RISC-V Embedded GCC toolchain.

This PR fixes #1887.

[ilg-ul]

BTW, the triplet used by Debian is riscv64-unknown-elf, I think it should be added too.

[ilg-ul]

I did a new test and the -march=rv32ima_zicsr_zifencei_zba_zbb_zbs_zbkb -mabi=ilp32 options used by the pico_riscv_gcc.cmake are accepted by the official RISC-V GCC 14.2 compiler.

zcmp is not accepted by my 14.2.0-1 release because of the outdated binutils 2.42 (the error comes from the assembler).

I did a new build with binutils 2.43.1 and the zcmp extension was accepted:

ilg@wksi tmp % /Users/ilg/Work/xpack-dev-tools-build/riscv-none-elf-gcc-14.2.0-2/darwin-x64/application/bin/riscv-none-elf-gcc m.c -march=rv32imac_zicsr_zifencei_zba_zbb_zbs_zbkb_zca_zcb_zcmp -mabi=ilp32
ilg@wksi tmp % 

I'll make a new 14.2.0-2 release with the latest binutils.

Updating the pico_riscv_gcc_zcb_zcmp.cmake file should no longer be a problem.