Use standard_output_file in a couple more places

Use standard_output_file in a couple more places

I noticed a few files left over in the testsuite/ directory in the
build tree after running tests.  Normally output files should not wind
up there, but instead should end up in the test's subdirectory.  This
isn't always crucial, but in the past there were problems with tests
clashing, preventing parallel runs, and so it is better to be clean
here.

This patch changes a couple of tests to use standard_output_file to
fix this problem.

Tested by re-running the tests in question and examining the
directory.

gdb/testsuite/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	* gdb.base/gnu-ifunc.exp (build): Use standard_output_file.
	* gdb.trace/unavailable-dwarf-piece.exp: Use standard_output_file.