[dev.link] cmd/link/internal/loader: add elf symbol methods

[dev.link] cmd/link/internal/loader: add elf symbol methods

Add new get/set methods to the loader for recording the ELF symbol
index for a given loader symbol. These are map-based, since it is
expected that many/most symbols will not need an ELF symbol table
entry.

Change-Id: I1102c3637775515ccc6650118e8b059468a2c3ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/230461
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>