Move [vdso] after exe/linker in _r_debug

gdbserver assumes that the first entry is the exe, so it must come
first.

Fixes debugging of executables with gdb.

Bug: https://issuetracker.google.com/112627083
Bug: http://b/110967431
Test: gdbclient.py -r toybox
Change-Id: I7b30398d679c3f8b92d8d02572f9073ae0fce798