arch-power: Add multi-mode support

This adds multi-mode support and allows the simulator to
read, interpret and execute 32bit and 64-bit, big and
little endian binaries in syscall emulation mode.

During process initialization, a minimal set of hardware
capabilities are also advertised by the simulator to show
support for 64-bit mode and little endian byte order.
This also adds some fixups specific to 64-bit ELF ABI v1
that readjust the entry point and symbol table due to the
use of function descriptors.

Change-Id: I124339eff7b70dbd14e50ff970340c88c13bd0ad
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40944
Reviewed-by: Boris Shingarov <shingarov@labware.com>
Maintainer: Boris Shingarov <shingarov@labware.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/loader/object_file.cc

@@ -66,6 +66,8 @@ archToString(Arch arch)
         return "thumb";
       case Power:
         return "power";
+      case Power64:
+        return "power64";
       case Riscv64:
         return "riscv64";
       case Riscv32:
@@ -84,6 +86,8 @@ opSysToString(OpSys op_sys)
       case Tru64:
         return "tru64";
       case Linux:
+      case LinuxPower64ABIv1:
+      case LinuxPower64ABIv2:
         return "linux";
       case Solaris:
         return "solaris";