Elaborate on ABI and gcc

[bugabinga]

Thank you for taking the time to create this awesome resource!

Here are two initial suggestions coming from the perspective of a programmer used to higher level languages:

1. Please explain the significance and meaning of an ABI some more. It was not obvious to me that the whole (programming) world depends on libc. Why are there several libc implementations per platform? Also shouldn't the implementation be irrelevant at this stage because the libc interface is the same per platform (is it?).

2. Why the sudden dependency on GCC in the rust toolchain? When learning rust it is explained that it uses llvm under the hood. This would imply the use of clang/lld , wouldn't it?