(PCINDEX): New macro.

(PCINDEX): New macro.
(md_relax_table): No longer const.  Add PCINDEX entries.
(m68k_ip): For AINDX with simple symbol operand, generate a PCINDEX frag if PC
is used, or do normal non-AINDX processing for address register.
(m68k_init_after_args): If cpu is 68000 or 68010, fix md_relax_table to prevent
relaxation of PCINDEX/BYTE mode to SHORT, since they don't support that mode.
(md_convert_frag_1, case PCLEA/LONG): Add 4 to offset, not 2.  Add support for
new PCINDEX modes.
(md_estimate_size_before_relax): Process PCINDEX/SZ_UNDEF mode.
(md_convert_frag_1, case PCLEA/SHORT): Add 2 to offset.
(m68k_ip, case most punctuation/AOFF): If using PC, call add_frag using PCLEA.