Add Cosmopolitan Libc Platform (#2598)

This PR adds the Cosmopolitan Libc platform enabling compatibility with multiple
x86_64 operating systems with the same binary. The platform is similar to the
Linux platform, but for now only x86_64 with interpreter modes are supported.

The only major change to the core is `posix.c/convert_errno()` was rewritten to use
a switch statement. With Cosmopolitan errno values depend on the currently
running operating system, and so they are non-constant and cannot be used in array
designators. However, the `cosmocc` compiler allows non-constant case labels in
switch statements, enabling the new version.

And updated wamr-test-suites script to add `-j <platform>` option. The spec tests
can be ran via `CC=cosmocc ./test_wamr.sh -j cosmopolitan -t classic-interp`
or `CC=cosmocc ./test_wamr.sh -j cosmopolitan -t fast-interp`.

product-mini/README.md

@@ -443,3 +443,23 @@ make
    aos make
    ```
    download the binary to developerkit board, check the output from serial port
+
+## Cosmopolitan Libc
+Currently, only x86_64 architecture with interpreter modes is supported.
+
+Clone the Cosmopolitan Libc. Setup `cosmocc` as described in [Getting Started](https://github.com/jart/cosmopolitan/#getting-started) being sure to get it into `PATH`.
+
+Build iwasm
+``` Bash
+export CC=cosmocc
+export CXX=cosmoc++
+rm -rf build
+mkdir build
+cmake -DWAMR_BUILD_INTERP=1 -DWAMR_BUILD_FAST_INTERP=1 -B build
+cmake --build build -j
+```
+
+Run like
+``` Bash
+./build/iwasm.com <wasm file>
+```