Fix several issues of document, spec test script and simd (#767)

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>

tests/wamr-test-suites/spec-test-script/runtest.py

@@ -902,9 +902,6 @@ def compile_wast_to_wasm(form, wast_tempfile, wasm_tempfile, opts):
             wast_tempfile, "-o", wasm_tempfile ]
 
     # optional arguments
-    if opts.simd:
-        cmd.append("--enable-simd")
-
     if opts.ref_types:
         cmd.append("--enable-reference-types")
         cmd.append("--enable-bulk-memory")