target/ppc: Implement lwsync with weaker memory ordering

This allows an x86 host to no-op lwsyncs, and ppc host can use lwsync
rather than sync.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220519135908.21282-5-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>