In order to do that, go to File->Add to Program... and pick this file from the file system. It will be added to ghidra. you might need to reanalyze the whole program. Afterward, you should find those functions in the function list and can view them in the decompile.
The second piece of code is a so-called PLT (program linkage table) thunk. It retrieves the pointer to the target function from the GOT (Global offset table) and jumps to it. The GOT pointer is usually filled in by the dynamic loader (ld.so or an analog) at the program's startup. So indeed, the real function should be in one of the dependent shared libraries....
Your host architecture has little to do with reverse engineering, unless you are trying to execute a binary. Decompilers/disassemblers are built to understand a variety of binary formats (not too different from a video player that handles many codecs). As long as your tools support the architecture, you're fine.
CGI is not a specific type of file; it more so describes the way the file is interacted with. A CGI file could be a script written in any scripting language (e.g. Python, Bash, Perl etc.), or it could be an ELF executable like you have here.
Since it's just a normal ELF, you can use any common disassembly/decompilation tool that you would use for other ...
To enable full relro:
What does this do? - it provides -z,relro,-z,now flag to linker as an argument. This enables full relro (notice -z,now flag).
Partial relro is enabled by default on modern gcc compilers.
How to disable relro? Pass following flag
Difference between full and partial relro: partial relro makes partial ....