lkdtm: Add more EFI runtime tests

Two bugs were found in the arm64 EFI runtime support that weren't caught
by LKDTM. The existing EFI_RUNTIME_CRASH test instructs the firmware to
write to kernel memory that is mapped read-only, which usually fails as
expected. However, this does not exercise the runtime mapping logic in
the kernel, nor the fault recovery logic when the faulting address is
inside the EFI runtime memory regions itself.

So add two more test cases:
- a write to R/O memory test that uses the firmware's own executable
  memory as an output buffer
- a branch to memory that is occupied by the firmware's runtime services
  function table, which is usually mapped non-execute.

This depends on the EFI memory attributes table having been exposed by
the firmware and having been taken into account by the runtime mapping
code, so this is used as a prerequisite by the new tests, otherwise it
produces an XFAIL.

Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

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