libgccjit: Support signed char flag

gcc/jit/ChangeLog:

	* dummy-frontend.cc (jit_langhook_init): Send flag_signed_char
	argument to build_common_tree_nodes.

gcc/testsuite/ChangeLog:

	* jit.dg/all-non-failing-tests.h: Add test-signed-char.c.
	* jit.dg/test-signed-char.c: New test.