Added gdb-remote auxv tests.

First batch of auxv-related tests from llgs branch.

Includes helpers for unescaping gdb-remote binary-escaped
data, converting binary data from inferior endian-ness to
integral values, etc.

Tests on debugserver are expected to be skipped since it
doesn't support auxv and the tests are geared to be skipped
on platforms that don't broadcast support for the feature
in qSupported.  (llgs is listed as XFAIL since qSupported
support in llgs upstream is not there, so the support check
cannot work in upstream llgs.)