g++.dg/warn/Wplacement-new-size-1.C, -2, -6: Fix for default_packed targets

g++.dg/warn/Wplacement-new-size-1.C, -2, -6: Fix for default_packed targets

Looking at commit de05c19d5fd6, that adjustment to these
tests apparently assumed that the testsuite is run (only) on
targets where structure memory layout has padding as per
"natural alignment".  For cris-elf, a target with no padding
in structure memory layout, these tests have been failing
since that commit.

Tested cris-elf and x86_64-linux, committed as obvious.

gcc/testsuite:
	* g++.dg/warn/Wplacement-new-size-1.C,
	g++.dg/warn/Wplacement-new-size-2.C,
	g++.dg/warn/Wplacement-new-size-6.C: Adjust for
	default_packed targets.