[gdb/testsuite] Handle remote host in gdb_load_shlib

With test-case gdb.arch/ftrace-insn-reloc.exp and host board
local-remote-host-notty and target board native-gdbserver I run into:
...
(gdb) tstart^M
Target returns error code '.In-process agent library not loaded in process.  \
  Fast and static trace points unavailable.'.^M
(gdb) FAIL: gdb.arch/ftrace-insn-reloc.exp: start trace experiment
...

Fix this by:
- handling remote host in gdb_load_shlib, and
- moving the gdb_load_shlib to after the clean_restart, such that the
  set solib-search-path can take effect.

Tested on x86_64-linux.