Updated gdbclient path support.

Now gdbclient accepts a fully qualified pathname for EXEs, which it will
not modify, or a relative pathname, to which it will prefix
"/system/bin".  As an example, each of the following now works.

Fully qualified:
adb shell gdbserver :5039 /system/bin/ping
bg
gdbclient /system/bin/ping :5039 /system/bin/ping

Relative:
adb shell gdbserver :5039 /system/bin/ping
bg
gdbclient ping :5039 /system/bin/ping

Change-Id: I1e4c9fca64c4fbc52c255271cc7f83f35c258509