debug/elf is the package to use. You'll want to call the Symbols
method and look through the Symbols for one whose Value is <= the PC
you want while Value+Size is > the PC you want.
If you are looking at runtime PC's from a stack trace, be aware that
on most systems these days programs are position-independent, so there
will be an offset between the addresses in the binary and the
addresses from the stack trace. I don't know offhand of a standard
way to figure out that offset.
Ian