explicitly state code assumptions on output section in AArch64 ld handlers

When support for non-contiguous memory was added, some corner cases when
sections were removed from the output object, did not emit fatal error and
reached code paths that correctly assumed every input section had a valid
output section, and this led to crashes due to segfault.

This patch adds BFD_ASSERTs in the previously segfaulting code, to
explicitly state code assumptions.