Remove downloading, guessing the host executable, the calls to
gdb_file_cmd and gdb_target_cmd, and "load" support.
(infer_host_exec): New function broken out from gdb_load.
(gdb_load): New wrapper for gdbserver_gdb_load.
* lib/mi-support.exp (mi_gdb_target_cmd): New function, based on
gdb_target_cmd. Use -target-select.
(mi_gdb_file_cmd): New function, broken out from mi_gdb_load.
Download binaries to the host. Clear last_mi_remote_file when
we load a new binary.
(mi_gdb_load): Call mi_gdb_file_cmd. If gdbserver.exp is loaded,
call gdbserver_gdb_load and mi_gdb_target_cmd.
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