`longjmp` can be called to raise `NoMemoryError` or to trigger GC when
`malloc` fails to allocate memory in `ruby_xmalloc` family. In such
case, `malloc` call in `longjmp` will fail again, and Asyncify unwinding
operation corrupts the memory space by using the failed pointer as
Asyncify buffer. This commit uses statically allocated buffer to avoid
such situation.

the original rb_wasm_setjmp implementation always unwinds to the root
call frame to have setjmp compatible interface, and simulate sjlj's
undefined behavior. Therefore, every vm_exec call unwinds to main, and
a deep call stack makes setjmp call very expensive. The following
snippet from optcarrot takes 5s even though it takes less than 0.3s on
native.

```
[0x0, 0x4, 0x8, 0xc].map do |attr|
  (0..7).map do |j|
    (0...0x10000).map do |i|
      clr = i[15 - j] * 2 + i[7 - j]
      clr != 0 ? attr | clr : 0
    end
  end
end
```

This patch adds a WASI specialized vm_exec which uses lightweight
try-catch API without unwinding to the root frame. After this patch, the
above snippet takes only 0.5s.

Tabs were expanded because the file did not have any tab indentation in unedited lines.
Please update your editor config, and use misc/expand_tabs.rb in the pre-commit hook.

configure.ac: setup build tools and register objects

main.c: wrap main with rb_wasm_rt_start to handle asyncify unwinds

tool/m4/ruby_wasm_tools.m4: setup default command based on WASI_SDK_PATH
environment variable. checks wasm-opt which is used for asyncify.

tool/wasm-clangw wasm/wasm-opt: a clang wrapper which replaces real
wasm-opt with do-nothing wasm-opt to avoid misoptimization before
asyncify. asyncify is performed at POSTLINK, but clang linker driver
tries to run optimization by wasm-opt unconditionally. inlining pass
at wasm level breaks asyncify's assumption, so should not optimize
before POSTLIK.

wasm/GNUmakefile.in: wasm specific rules to compile objects