Fix some issues on MacOS platform (#937)

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>

core/shared/platform/common/posix/posix_memmap.c

@@ -16,6 +16,7 @@ static size_t total_size_munmapped = 0;
 
 #define HUGE_PAGE_SIZE (2 * 1024 * 1024)
 
+#if !defined(__APPLE__) && !defined(__NuttX__)
 static inline uintptr_t
 round_up(uintptr_t v, uintptr_t b)
 {
@@ -29,6 +30,7 @@ round_down(uintptr_t v, uintptr_t b)
     uintptr_t m = b - 1;
     return v & ~m;
 }
+#endif
 
 void *
 os_mmap(void *hint, size_t size, int prot, int flags)