Added PLATFORM_LINUX_FORCE_LLGS_LOCAL environment variable to turn on llgs-local by default.

If the user sets PLATFORM_LINUX_FORCE_LLGS_LOCAL, it will force the default behavior of
using local-llgs to be true.

This allows setting the environment variable for test runs.  The .lldbinit mechanism to flip
the switch doesn't seem to work.

Do this:
```
export PLATFORM_LINUX_FORCE_LLGS_LOCAL 1
```
and then run tests to get local llgs behavior.