Adds a new option -xarch_root which sets a path that is prepended to:
- The application executable's interpreter, if the original does not exist.
- SYS_openat paths, if the original does not exist.
- System paths ued for loading private libraries: here the prefix is prepended before checking whether the original exists.
Splits dynamorio_app_init() into two pieces in order to have the options set up at the time the loader maps the interpreter, while avoiding ordering problems with the rest of the initialization.
The new option also auto-sets -ignore_takeover_timeout for convenience, as that is always needed when running under QEMU.
Manually tested in cross-compile AArchXX setups on a Debian system. Test suite integration is forthcoming.
Issue: #4719