gdbclient.py: don't use make to dump target info.

make dumpvar takes forever (~0.45s per variable on my machine), and the
information we want from it is already available in the environment.
Construct the paths we need ourselves, instead of relying on make to do
it for us, shaving off about a second from a gdbclient.py invocation.

Test: gdbclient.py -r ls
Change-Id: I8c563da726c8f61a23e2eb701f2f272a4c71d713