ARROW-13426: [C++][Gandiva] Use platform pointer sizes in code generation

[pablosichert]

Hey there!

This PR uses pointers in the JIT code generation / function invocation instead of the hard-coded int64_t type, which especially breaks when indexing into pointers to pointers on platforms where the pointer size is not 64 bit wide.

[github-actions[bot]]

https://issues.apache.org/jira/browse/ARROW-13426

[pitrou]

Is it worth keeping this open/reviving it? cc @js8544

[js8544]

Is it worth keeping this open/reviving it? cc @js8544

This feature is definitely worth having, if the author would like to resolve the conflicts.

[amol-]

Closing because it has been untouched for a while, in case it's still relevant feel free to reopen and move it forward 👍