Fix the PR22983 test so that it will work regardless of the order of the symbols...

Fix the PR22983 test so that it will work regardless of the order of the symbols in the dynamic symbol table.

See email thread starting here for more details:
  https://sourceware.org/ml/binutils/2018-06/msg00036.html

	PR 22983
	* testsuite/ld-plugin/lto.exp: Use individual tests to check for
	the presence of each expected symbol.
	* testsuite/ld-plugin/pr22983.1.d: New file.
	* testsuite/ld-plugin/pr22983.2.d: New file.
	* testsuite/ld-plugin/pr22983.3.d: New file.
	* testsuite/ld-plugin/pr22983.4.d: New file.