[gdb/testsuite] Fix have_avx for remote target

[gdb/testsuite] Fix have_avx for remote target

In proc have_avx we compile some source into an exec, resulting in a file $obj
on build, and then attempt to execute it on target:
...
    set result [remote_exec target $obj]
...

Fix this by using gdb_remote_download target.

Likewise in a few other procs that use "remote_exec target".

Tested on x86_64-linux.