Add build instructions for 64-bit Raspbian

[SeanTAllen]

Closes #3872

[SeanTAllen]

This should be updated with the instructions that include whatever comes out of #3870 before merging.

[chalcolith]

#3949 automatically sets -fPIC for aarch64, so no changes are needed.

[SeanTAllen]

So, we don't currently use -fPIC on gravitron so that would be a change there @kulibali. I don't think we need for all aarch64. Is gravitron not aarch64? I don't think we want that on for gravitron if its not needed. At least not as an "always on".

[chalcolith]

I haven't been able to find a way to detect Raspbian from cmake or the shell -- still trying to figure out how.

[SeanTAllen]

@kulibali I personally think it would be fine to have build instructions that say "hey for X, you need to use Y" and leave the raspbian64 at that.

[chalcolith]

OK, I have updated #3949 to not try to change the pic flag, so the instructions should say to use make libs pic_flag=-fPIC and make configure pic_flag=-fPIC.

[SeanTAllen]

@kulibali so pic_flag=-fPIC isn't needed on the build command?

[chalcolith]

@SeanTAllen correct; the build flags are baked into makefiles by make configure, and make build calls make on those makefiles.