tcg/ppc: disable atomic write check on ppc32

This fixes building for ppc64 on ppc32 (changed in 5964fca8a12c):

tcg/ppc/tcg-target.inc.c: In function 'tb_target_set_jmp_target':
include/qemu/compiler.h:86:30: error: static assertion failed: \
  "not expecting: sizeof(*(uint64_t *)jmp_addr) > ATOMIC_REG_SIZE"
	QEMU_BUILD_BUG_ON(sizeof(*ptr) > ATOMIC_REG_SIZE); \
	^
tcg/ppc/tcg-target.inc.c:1377:9: note: in expansion of macro 'atomic_set'
	atomic_set((uint64_t *)jmp_addr, pair);
	^

Suggested-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170911204936.5020-1-f4bug@amsat.org>
[rth: Added commentary requested by pmm.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
