[llvm-objcopy] Make ihex writer similar to binary writer

There is no need to differentiate whether `UseSegments` is true or
false. Unifying the cases makes the behavior closer to BinaryWriter.

This improves compatibility with objcopy because SHF_ALLOC sections not in
a PT_LOAD will not be skipped. Such cases are usually erroneous input, though.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D104186