Fix document issues: add ARC to supported targets, fix how to build wamrc for MacOS.
Fix spec case test script issue: the latest wabt has enabled simd by default, no need to
add "--enable-simd" option for test script.
Fix simd LLVM IR compilation issue: using index calculated by opcode to access array
element should not be out of array boundary, add bh_assert() for it.
Signed-off-by: Wenyong Huang <wenyong.huang@intel.com>
