Update MJIT doc about bindgen and platforms [ci skip]

Author: k0kubun

doc/mjit.md

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+# MJIT
+
+This document has some tips that might be useful when you work on MJIT.
+
+## Supported platforms
+
+The following platforms are either tested on CI or assumed to work.
+
+* OS: Linux, macOS
+* Arch: x86\_64, aarch64, arm64, i686, i386
+
+### Not supported
+
+The MJIT support for the following platforms is no longer maintained.
+
+* OS: Windows (mswin, MinGW), Solaris
+* Arch: SPARC
+
+### Architectures
+
+## Bindgen
+
+If you see an "MJIT bindgen" GitHub Actions failure, please commit the `git diff` shown on the failed job.
+
+Refer to the following instructions for doing the same thing locally.
+Similar to `make yjit-bindgen`, `make mjit-bindgen` requires libclang.
+See also: [mjit-bindgen.yml](../.github/workflows/mjit-bindgen.yml)
+
+macOS seems to have libclang by default, but I'm not sure how to deal with 32bit architectures.
+For now, you may generate c\_64.rb with a 64bit binary, and then manually modify c\_32.rb accordingly.
+
+### x86\_64-linux
+
+```sh
+sudo apt install \
+  build-essential \
+  libssl-dev libyaml-dev libreadline6-dev \
+  zlib1g-dev libncurses5-dev libffi-dev \
+  libclang1
+./autogen.sh
+./configure --enable-yjit=dev_nodebug --disable-install-doc
+make -j
+make mjit-bindgen
+```
+
+### i686-linux
+
+```sh
+sudo dpkg --add-architecture i386
+sudo apt install \
+  crossbuild-essential:i386 \
+  libssl-dev:i386 libyaml-dev:i386 libreadline6-dev:i386 \
+  zlib1g-dev:i386 libncurses5-dev:i386 libffi-dev:i386 \
+  libclang1:i386
+./autogen.sh
+./configure --disable-install-doc
+make -j
+make mjit-bindgen
+```
+
+Note that you cannot run x86\_64 bindgen with an i686 binary, and vice versa.
+Also, when you install libclang1:i386, libclang1 will be uninstalled.
+You can have only either of these at a time.
+
+## Local development
+
+### Always run make install
+
+Always run `make install` before running MJIT. It could easily cause a SEGV if you don't.
+MJIT looks for the installed header for security reasons.
+
+### --mjit-debug vs --mjit-debug=-ggdb3
+
+`--mjit-debug=[flags]` allows you to specify arbitrary flags while keeping other compiler flags like `-O3`,
+which is useful for profiling benchmarks.
+
+`--mjit-debug` alone, on the other hand, disables `-O3` and adds debug flags.
+If you're debugging MJIT, what you need to use is not `--mjit-debug=-ggdb3` but `--mjit-debug`.