powerpc/64s: Disable pcrel code model on Clang

Clang has a bug that casues the pcrel code model not to be used when any of
-msoft-float, -mno-altivec, or -mno-vsx are set. Leaving these off causes
FP/vector instructions to be generated, causing crashes. So disable pcrel
for clang for now.

Fixes: 7e3a68be

 ("powerpc/64: vmlinux support building with PCREL addresing")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230426055848.402993-3-npiggin@gmail.com