[lldb/test] Fix TestScriptedProcess.py failures on arm linux bots

This patch tentatively fixes TestScriptedProcess.py which has been
failing on the `lldb-arm-ubuntu` & `lldb-aarch64-ubuntu` bots:

- https://lab.llvm.org/buildbot/#/builders/17/builds/44965
- https://lab.llvm.org/buildbot/#/builders/96/builds/48152



According to the test log, on those systems, the clang driver that build
the test binary doesn't have the `-m` flag to specify the architure so
this patch replaces it with the `-target` flag using `clang -dumpmachine`
to get the host triple.

Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>