powerpc/64s: Use .machine power4 around dcbt

There are multiple decodings for the "dcbt" mnemonic, so the assembler
has to pick one.

That requires passing -many to the assembler, which is not recommended.

Without -many the clang 14 / binutils 2.38 build fails with:

  arch/powerpc/kernel/exceptions-64s.S:2976: Error: junk at end of line: `0b01010'
  clang: error: assembler command failed with exit code 1 (use -v to see invocation)

Fix it by adding .machine directives around the use of dcbt to specify
which encoding is desired.

Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240229122521.762431-2-mpe@ellerman.id.au