AArch64: Add FLAG for tbl/tbx intrinsics [PR94442]

2020-11-10  Zhiheng Xie  <xiezhiheng@huawei.com>
	    Nannan Zheng  <zhengnannan@huawei.com>

gcc/ChangeLog:

	* config/aarch64/aarch64-simd-builtins.def: Add proper FLAG
	for tbl/tbx intrinsics.