Fix overflow detection in the Z80 assembler.

 * config/tc-z80.c (emit_data_val): Warn on constant overflow.
 (signed_overflow): New function.
 (unsigned_overflow): New function.
 (is_overflow): Use new functions.
 (md_apply_fix): Use signed_overflow.
 * testsuite/gas/z80/ez80_adl_suf.d: Fix test.
 * testsuite/gas/z80/ez80_isuf.s: Likewise.
 * testsuite/gas/z80/ez80_z80_suf.d: Likewise.