powerpc: Disable CPU unknown by CLANG when CC_IS_CLANG

powerpc: Disable CPU unknown by CLANG when CC_IS_CLANG

CLANG only knows the following CPUs:

generic, 440, 450, 601, 602, 603, 603e, 603ev, 604, 604e, 620, 630,
g3, 7400, g4, 7450, g4+, 750, 8548, 970, g5, a2, e500, e500mc, e5500,
power3, pwr3, power4, pwr4, power5, pwr5, power5x, pwr5x, power6,
pwr6, power6x, pwr6x, power7, pwr7, power8, pwr8, power9, pwr9,
power10, pwr10, powerpc, ppc, ppc32, powerpc64, ppc64, powerpc64le,
ppc64le, futur

Disable other ones when CC_IS_CLANG.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e62892e32c14a7a5738c597e39e0082cb0abf21c.1675335659.git.christophe.leroy@csgroup.eu