Fix 110803: use of plain char instead of signed char

Fix 110803: use of plain char instead of signed char

So the problem here is that plain char can either be signed
or unsigned depending on the target (powerpc and aarch64 are
unsigned while most other targets are signed). So the testcase
gcc.c-torture/execute/pr109986.c was assuming plain char was signed
char which is wrong so it is better to just change the `char` to be
`signed char`.
Note gcc.c-torture/execute/pr109986.c includes gcc.dg/tree-ssa/pr109986.c
where the plain char was being used.

Committed as obvious after a quick test to make sure gcc.c-torture/execute/pr109986.c
now passes and gcc.dg/tree-ssa/pr109986.c still passes.

gcc/testsuite/ChangeLog:

	PR testsuite/110803
	* gcc.dg/tree-ssa/pr109986.c: Change plain char to be
	`signed char`.