[gdb/contrib] cc-with-tweaks.sh: Create .dwz file in .tmp subdir

[gdb/contrib] cc-with-tweaks.sh: Create .dwz file in .tmp subdir

When running a test-case gdb.base/foo.exp with cc-with-dwz-m, a file
build/gdb/testsuite/outputs/gdb.base/foo/foo.dwz will be created, alongside
executable build/gdb/testsuite/outputs/gdb.base/foo/foo.

This can cause problems in f.i. test-cases that test file name completion.

Make these problems less likely by moving foo.dwz to a .tmp subdir:
build/gdb/testsuite/outputs/gdb.base/foo/.tmp/foo.dwz.

Tested on x86_64-linux.

gdb/ChangeLog:

2019-09-29  Tom de Vries  <tdevries@suse.de>

	* contrib/cc-with-tweaks.sh (get_tmpdir): New function.
	Use $tmpdir/$(basename "$output_file").dwz instead of
	"${output_file}.dwz".

gdb/testsuite/ChangeLog:

2019-09-29  Tom de Vries  <tdevries@suse.de>

        * gdb.dwarf2/gdb-index.exp: Handle new location of .dwz file.