libcxx: Disable llvm libunwind on ppc
libcxx: Disable llvm libunwind on ppc

It fails to build

libunwind/src/UnwindRegistersSave.S:561: Error: unrecognized opcode: `extern'

Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

kraj/meta-clang,kraj/meta-clang,kraj/meta-clang