[WIP] add linux prebuilt library

[yjh0502]

add linux prebuilt library. It built with musl-gcc in linux environment to ensure portability. (no dependency on glibc). Here's an ldd result.

$ ldd curl/prebuilt/linux/lib/libcurl.so
        linux-vdso.so.1 (0x00007fff3a5fd000)
        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f11bb595000)
        libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f11bb579000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f11bb387000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f11bba11000)

I haven't setup a unity project yet. If you are comfortable with the PR, I'll prepare remaining steps (add libcurl.so to Plugins directory, setup appropriate platform flags on asmdef, ...)

[Jayatubi]

The non-mobile platform prebuilt libs were designed to work for the Unity Editor but they should also work for standalone builts. It would be better if you add the Unity stuffs, such asmdef files, and run the test scene on Linux Unity Editor.

It is sorry that I've already left this organization thus I can't approve this PR for you 😜.