c-family: One more 40752 tweak for unsigned char.

c-family: One more 40752 tweak for unsigned char.

My last patch didn't fix all the failures on unsignd char targets.  We were
missing one warning because by suppressing -Wsign-conversion for the second
operand of + we missed an overflow that we want to warn about, and we
properly don't warn about unsigned / or %.

	PR testsuite/93391 - PR 40752 test fails with unsigned plain char.
	* c-warn.c (conversion_warning): Change -Wsign-conversion handling.
	* lib/target-supports.exp (check_effective_target_unsigned_char):
	New.