powerpc/boot/wrapper: Add "-z notext" flag to disable diagnostic

The "-z notext" flag disables reporting an error if DT_TEXTREL is set.

  ld.lld: error: can't create dynamic relocation R_PPC64_ADDR64 against
    symbol: _start in readonly segment; recompile object files with
    -fPIC or pass '-Wl,-z,notext' to allow text relocations in the
    output
  >>> defined in
  >>> referenced by crt0.o:(.text+0x8) in archive arch/powerpc/boot/wrapper.a

The BFD linker disables this by default (though it's configurable in
current versions). LLD enables this by default. So we add the flag to
keep LLD from emitting the error.

Signed-off-by: Bill Wendling <morbo@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201120224034.191382-2-morbo@google.com