Handle start section

[Luukdegram]

Although we do not need to handle the merging of start sections (that would be a linking error), we may have to re-index the start section to point to the new index of a function.

For now, we're not going to verify if we're building an executable or library. We'll handle that once we merge into zld.

[Luukdegram]

Implemented in 247f8ab