[LLD][ELF][ARM] Tidy up test to hook up missing filecheck patterns [NFC]

A couple of filecheck patterns had not been hooked up with
the patterns suffering from some drift. As this test is old
and llvm-objdump has improved a lot, take this opportunity to
hide the instruction encoding. I've also taken out a lot of
the explanatory comments that llvm-objdump improvements make
redundant, as these comments oftern don't get updated when addresses
change.

Differential Revision: https://reviews.llvm.org/D104907