opcodes: reflect register name changes for ARC64

[artemiy-volkov]

currently, arc64-elf-objdump is unaware of general register changes neither in arc32 nor arc64. For example, it still thinks that r60 is lp_count and not a limm register:

# cat 1.s
pushl_s r60

# arc64-elf-as 1.s                
# arc64-elf-objdump -d a.out

a.out:     file format elf64-littlearc64


Disassembly of section .text:

0000000000000000 <.text>:
   0:   c4ef                    pushl_s lp_count

Update regmods array to reflect changes to general registers. Alternatively, do away with regmods and just use regnames to store all general register name information.