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>

product-mini/platforms/darwin/CMakeLists.txt

@@ -55,8 +55,8 @@ if (NOT DEFINED WAMR_BUILD_LIBC_BUILTIN)
 endif ()
 
 if (NOT DEFINED WAMR_BUILD_LIBC_WASI)
-  # Disable libc wasi support by default
-  set (WAMR_BUILD_LIBC_WASI 0)
+  # Enable libc wasi support by default
+  set (WAMR_BUILD_LIBC_WASI 1)
 endif ()
 
 if (NOT DEFINED WAMR_BUILD_FAST_INTERP)
@@ -65,7 +65,7 @@ if (NOT DEFINED WAMR_BUILD_FAST_INTERP)
 endif ()
 
 if (NOT DEFINED WAMR_BUILD_MULTI_MODULE)
-  # Enable multiple modules
+  # Disable multiple module by default
   set (WAMR_BUILD_MULTI_MODULE 0)
 endif ()