Optimize gcc

[Runji Wang]

Thanks to @jiegec, x86_64-linux-musl-gcc is now working on rCore! 🎉

But now it takes nearly 10s (on QEMU) to build a simple C source file.
It's time to find the bottleneck and do some optimization!

• implement delay mapping files
• implement vfork

[Chen]

We need to implement CoW fork.

[Shengqi Chen]

Optimizing gcc is a really difficult work to do!