Set compilers/toolchain globally

[lem0nify]

Is your feature request related to a problem? Please describe.

I prefer to use clang/clang++ whenever possible. So it makes sense for me to set them as the default C and C++ compilers to avoid having to run xmake f --toolchain=llvm -c for every project, but I haven't found that option.

Describe the solution you'd like

• xmake g --toolchain=llvm once per life
• or config file somewhere in ~/.config

Describe alternatives you've considered

No response

Additional context

No response

[waruqi]

set_toolchains("llvm")