assembler_thumb_test: use 'objdump -D -M force-thumb' to disassemble.

Newer version of objcopy cannot strip '$d' from an object file, as it is
implicitly marked as 'precious'. This is intentional, introduced in the
upstream binutils repository by this commit:
- https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=fca2a38fdb391f810e309a12d5279047d4edac34

Since stripping of '$d' was being done only so we can disassemble
the object file with 'objdump -d', switch to using 'objdump -D -M
force-thumb' to force disassembly as thumb instructions.

Bug: None
Test: m test-art-host-gtest-assembler_thumb_test
Change-Id: I815f9f3a7949bdc49d872c9b726c87b3199b66ca