[gdb/testsuite] Use cc-with-tweaks.exp in dwarf4-gdb-index.exp

[gdb/testsuite] Use cc-with-tweaks.exp in dwarf4-gdb-index.exp

Board file dwarf4-gdb-index.exp contains all the commands from
cc-with-tweaks.exp (with CC_WITH_TWEAKS_FLAGS set to "-i").

Make dwarf4-gdb-index.exp smaller by including cc-with-tweaks.exp.

Tested on x86_64-linux for gdb.base.

gdb/testsuite/ChangeLog:

2019-05-01  Tom de Vries  <tdevries@suse.de>

	* boards/dwarf4-gdb-index.exp: Use cc-with-tweaks.exp.