h8300-elf-ld --relax, support for MOV instructions with addressing mode "register indirect with displacement"

[Michael Schewe]

The H8300 linker currently only relaxes MOV instructions with addressing modes "absolute address".
MOV instructions with addressing mode "register indirect with displacement" can also be relaxed from 24/32bit displacement to 16 bit displacement.
GCC often emits these instructions for switch encoding, table, array or struct accesses etc.
I would like to add support for these and some testcases too. Is this OK ?

Michael