ruyi venv 多工具链支持

[weilinfox]

需求描述

一些工程的构建需要多种工具链的支持，如 duo-buildroot-sdk 同时需要 gnu-milkv-milkv-duo-musl-bin 和 gnu-milkv-milkv-duo-elf-bin 支持构建。

• 这些工具链 target tuples 各不相同，没有文件冲突
• 至多存在一个 sysroot，或手动指定使用哪个工具链的 sysroot

[xen0n]

Usage:

ruyi venv -t gnu-milkv-milkv-duo-bin -t gnu-milkv-milkv-duo-musl-bin milkv-duo /tmp/venv

# the target tuples are riscv64-unknown-linux-gnu and riscv64-unknown-linux-musl
# these files/dirs are created:
#
# - meson-cross.$TARGET.ini
# - toolchain.$TARGET.cmake
# - sysroot.$TARGET

# the first toolchain given is the "primary" one setting the "primary target"
# files for the primary target are symlinked to the respective unmarked names
# e.g. sysroot -> sysroot.riscv64-unknown-linux-gnu