testsuite: fix dejagnu typos with underscores

Fix typos in dejagnu 'dg-*' directives with erroneous underscores like
'dg_'.

gcc/testsuite/ChangeLog:
	PR debug/30161
	PR c++/91826
	PR c++/116846

	* g++.dg/debug/dwarf2/template-func-params-7.C: Fix errant underscore.
	Cleanup whitespace in directives too.
	* g++.dg/lookup/pr91826.C: Fix errant underscore.
	* g++.dg/modules/indirect-1_b.C: Ditto.
	* gcc.target/powerpc/vsx-builtin-msum.c: Ditto.