arm: fix symbols around the .ARM.exidx section

Insert a direct assignment to the location counter to ensure that orphaned
sections cannot be emitted between the _exidx_start symbol and the .ARM.exidx
section.

Discussed with: jrtc27
MFC after: 1 week

(cherry picked from commit 1701dfae1be3be6f8f1fdd3b6b636d3161b33420)