[lldb][NFC] Try to adapt Cortex-M API test for an Unbuntu bot
When I added support for the Cortex-M exception return unwinding,
I got CI test failures on the lldb-remote-linux-ubuntu bot. The
triple from my test `binary.json`, "armv7m-apple", was not being
used for the Target, so the incorrect SysV / AAPCS ABI was being
selected, and that ABI plugin has default unwind plans that hardcode
the arm-code r11 frame pointer behavior. This is a Cortex-M
core, and r7 should be used. The Darwin Arm ABI plugin uses r7 for
both arm and thumb, and behaves correctly.
Try getting the triple from `binary.json` in the API test, creating
the target with that triple explicitly before loading the corefile.
This may help prevent however we were losing the "-apple-" part of
the triple. We'll see what the CI bot looks like with this added.