Revert "[LLD][ELF][ARM] Do not substitute BL/BLX for non STT_FUNC symbols."

There are still problems after the fix in
"[ELF][ARM] Fix regression of BL->BLX substitution after D73542"
so let's revert to get trunk back to green while we investigate.
See https://reviews.llvm.org/D73542

This reverts commit 5461fa2b.
This reverts commit 0b4a047b.