Fix gdb.base/all-architectures-2.exp failures

Fix gdb.base/all-architectures-2.exp failures

print_insn_amdgcn fails to report an error if the memory is not mapped.

disassemble 0x0,+4 returns:
Dump of assembler code from 0x0 to 0x4:
   0x0000000000000000:  v_cndmask_b32_e32 v0, s0, v0, vcc

instead of:
Dump of assembler code from 0x0 to 0x4:
   0x0000000000000000:  Cannot access memory at address 0x0

print_insn_amdgcn should return -1 if target_read returns 0 or
TARGET_XFER_E_IO. Also, use TARGET_OBJECT_CODE_MEMORY since we are
disassembling code memory.

gdb/ChangeLog:

        * amdgcn-rocm-tdep.c (print_insn_amdgcn): Check target_read's
        return value and return -1 if it is 0 or TARGET_XFER_E_IO.
        (amdgcn_rocm_displaced_step_fixup): Fix indentation.
        (amdgcn_gdbarch_init): Fix indentation.

Change-Id: Iff607a45efd0d369d44e7a8b34c8eb612985b5de