[ELF][ARM] Simplify some llvm-objdump tests with both ARM/Thumb states

llvm-objdump can switch between ARM/Thumb states after D60927.

In a few lld tests, we run both

* llvm-objdump -d -triple=thumbv7a-none-linux-gnueabi %t
* llvm-objdump -d -triple=armv7a-none-linux-gnueabi %t

to test ARM/Thumb parts of the same file. In many cases we can just
run one command. There is a problem that prevents us from cleaning
more tests (e.g. test/ELF/arm-thumb-interwork-thunk.s):

In llvm-objdump, while we have ARM/Thumb (primary and secondary)
MCDisassembler and MCSubtargetInfo, we have just one MCInstrAnalysis
which is used to resolve the targets of calls in both ARM/Thumb parts.

    // ThumbMCInstrAnalysis evaluating ARM parts or ARMMCInstrAnalysis evaluating Thumb parts
    // will have incorrect offsets.
    // An example of llvm-objdump -d -triple=thumbv7a on ARM part:
    1304: 3d ff ff fa  blx     #-780                 # no <...>
    1308: 06 00 00 ea  b       #24 <arm_caller+0x24> # wrong target due to wrong offset

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D66539

llvm-svn: 369535