[kernel] E: Improve readability of build.rs

[LucasGdosR]

This splits the main function according to nanvix/nanvix#385

Most of it was copy-paste. Some points stand out:

• setup_toolchain() used &str. I could convert it to String to return it, but I figured using lifetime annotations would be better to avoid allocation and keep the code unchanged.
• compile_source() could take references & to asm_sources and cflags, but I moved ownership since they're used only once. I passed a reference to out_dir, since it gets used by the other functions.
• I removed the variable cc, which was used only once. Instead, "gcc".to_string() is called directly in compile_source(). If it were allocated inside setup_toolchain() and returned, it would add yet another argument to compile_source(), or a tuple packing cc and cflags, which would be less readable IMO.

[ppenna]

Please add a rustdoc comment to each function in this file. This will help other newcomers to understand what is going on.

Also, could you add the with the following sections to the rustdoc comment:

• Description
• Parameters
• Return Value

This will ensure consistency with other source files in the project.

.

[LucasGdosR]

I have added the rustdoc comment to each function. I also included a struct along with its rustdoc. With this struct, gcc is specified inside the setup_toolchain() function, where it belongs.