Fix gdb.threads/dlopen-libpthread.exp crash

Fix gdb.threads/dlopen-libpthread.exp crash

Following, commit

  6e774b13c3b81ac2599812adf058796948ce7e95 Make ftrace tests work with remote targets

the test gdb.threads/dlopen-libpthread.exp started to fail with:

  ERROR: error copying "/home/emaisin/build/binutils-gdb/gdb/testsuite/gdb.threads/dlopen-libpthread.so": no such file or directory

This is because the library path is not computed using
standard_output_file, so we try to gdb_load_shlibs an unexisting file.

gdb/testsuite/ChangeLog:

	* gdb.threads/dlopen-libpthread.exp: Set binfile_lib using
	standard_output_file.  Remove unused binfile variable.