Arc: Fix write_pc for Opella-XD

Recently added function arc_elf_write_pc() had an error in case target is
Opella-XD, because this function has been using a hardcoded PC regnum, but
in case of Opella-XD gdbserver PC register has different regnum. This commit
fixes this, now PC will always be written to the regnum defined in struct
gdbarch. The same is done for the STATUS32 register in this function.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>