[llvm-jitlink] Add an explicit -debugger-support option.

Commit 69be352a restricted the MachO debugger support testcase to run on
Darwin only, but we still need to disable debugger support by default for
other noexec tests.

This patch introduces a -debugger-support option to llvm-jitlink that is
on-by-default when executing code, and off-by-default for noexec tests. This
should prevent regression tests from trying (and failing) to set up MachO
debugging support when running on non-Darwin platforms.

to explicitly enable/disable support.