MINOR: sym: add helpers to collect the symbols of an ELF image

MINOR: sym: add helpers to collect the symbols of an ELF image

The next commits read symbols from ELF images (an embedded compressed table
and the object's file), and both need the same primitives, so introduce
them here: validate an ELF header and its section header table, keep every
read inside the image, find .symtab and its string table, and collect its
function symbols into the builder, applying the object's load address.
Parsing an ELF image needs no help from the dynamic linker, so they are
gated on __ELF__ alone; only enumerating the loaded objects needs
dl_iterate_phdr().

They are tagged __maybe_unused because their users come in later commits,
which may be picked independently.