Fix some issues on MacOS platform
- Enable libc-wasi by default
- Set target abi to "gnu" if it is not set for wamrc to avoid generating
object file of unsupported Mach-O format
- Set `<vendor>-<sys>` info according to target abi for wamrc to support
generating AOT file for other OSs but not current host
- Set cpu name if arch/abi/cpu are not set to avoid checking SIMD
capability failed
- Set size level to 1 for MacOS/Windows platform to avoid relocation type
unsupported warning
- Clear posix_memmap.c compiling warning
- Fix spec case test script issues, enable test spec cases on MacOS
Signed-off-by: Wenyong Huang <wenyong.huang@intel.com>