* as.c (perform_an_assembly_pass): If using cgen, call gas_cgen_begin.

* as.c (perform_an_assembly_pass): If using cgen, call gas_cgen_begin.
* cgen.c (gas_cgen_begin): New function.  If flag_signed_overflow_ok is set
call cgen_set_signed_overflow_ok otherwise call cgen_clear_signed_overflow_ok.
* cgen.h: Prototype gas_cgen_begin.
* testsuite/gas/m32r/m32r.exp: Run signed-relocs test.
* testsuite/gas/m32r/signed-relocs.s: New file: Test signed relocs.
* testsuite/gas/m32r/signed-relocs.d: New file: Expected results