Inconsistency in RootFS handling between bare ELFs and shebangs

[dnadlinger]

As mentioned on Discord the other day, there is an inconsistency in how the binary lookup in LinuxSyscalls is handled between actual ELF binaries and files with a shebang.

For the execve()/… path, the RootFS path is only prepended to absolute paths if the file actually exists there:

https://github.com/FEX-Emu/FEX/blob/8ad14728f6e4b5da4d1a42a38910785aa18802c7/Source/Tests/LinuxSyscalls/Syscalls.cpp#L122-L131

For actual ELF binaries, this is the only path involved. If that file starts with a shebang, however, the RootFS path is prepended unconditionally to the path specified in the shebang, no matter whether the file actually exists in the RootFS or not:

https://github.com/FEX-Emu/FEX/blob/8ad14728f6e4b5da4d1a42a38910785aa18802c7/Source/Tests/LinuxSyscalls/Syscalls.cpp#L98-L103

(same in FEXLoader)

This seems odd, and breaks use cases where most of the x86_64 files actually do exist on the host (with proper paths), and only few "emulated" binaries are to be read from the RootFS instead.

Additionally, part of the shebang handling logic is duplicated between FEXLoader and LinuxSyscalls, which might sensibly be merged:

https://github.com/FEX-Emu/FEX/blob/8ad14728f6e4b5da4d1a42a38910785aa18802c7/Source/Tests/FEXLoader.cpp#L125-L155

https://github.com/FEX-Emu/FEX/blob/8ad14728f6e4b5da4d1a42a38910785aa18802c7/Source/Tests/LinuxSyscalls/Syscalls.cpp#L74-L103

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I was working on a fix for this, until @Sonicadvance1 pointed out that in situations where the RootFS isn't really necessary (e.g. when using Nix with binfmt_misc or similar, where binaries for both architectures exist side-by-side on the host and paths are properly set up for this), I might just as well disable the RootFS altogether. Still, it seems that the inconsistency here is unexpected behaviour – I am happy to finish this up and open a PR if desired.