powerpc: Fix fatal warnings flag for LLVM's integrated assembler

When building with LLVM_IAS=1, there is an error because
'-fatal-warnings' is not recognized as a valid flag:

  clang: error: unsupported argument '-fatal-warnings' to option '-Wa,'

Use the double hyphen version of the flag, '--fatal-warnings', which
works with both the GNU assembler and LLVM's integrated assembler.

Fixes: 608d4a5c ("powerpc: Error on assembly warnings")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240405-ppc-fix-wa-fatal-warnings-clang-v1-1-bdcd969f2ef0@kernel.org