Rewrite symbol_inject to be testable

Parse the machine-specific symbol tables into a custom symbol table
object and use a single function to find the offset and size of the
symbol in the custom table.  Make the functions to convert a
machine-specific symbol table into the custom table also take mocks,
and provide functions to dump mocks from real files.  Add tests
that take the mocks and verify they produce the right offset and
size.

Test: cmd/symbol_inject/*_test.go
Change-Id: I25654032b5017dd13a9a1fe29f8b8826ce5bc82b