OvmfPkg: Skip initrd command on Xcode toolchain

OvmfPkg: Skip initrd command on Xcode toolchain

OVMF booting stops with the assert if built with Xcode on macOS:

  Loading driver at 0x0001FAB8000 EntryPoint=0x0001FABF249 LinuxInitrdDynamicShellCommand.efi
  InstallProtocolInterface: BC62157E-3E33-4FEC-9920-2D3B36D750DF 1F218398
  ProtectUefiImageCommon - 0x1F218140
    - 0x000000001FAB8000 - 0x0000000000008A60

  ASSERT_EFI_ERROR (Status = Unsupported)
  ASSERT LinuxInitrdDynamicShellCommand.c(378): !EFI_ERROR (Status)

The assert comes from InitializeHiiPackage() after an attempt to
retrieve HII package list from ImageHandle.

Xcode still doesn't support HII resource section and
LinuxInitrdDynamicShellCommand depends on it. Likewise 277a3958d93a
("OvmfPkg: Don't include TftpDynamicCommand in XCODE5 tool chain"),
disable initrd command if built with Xcode toolchain

Fixes: ec41733cfd10 ("OvmfPkg: add the 'initrd' dynamic shell command")
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200514134820.62047-1-r.bolshakov@yadro.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>