install target disrespects PREFIX and installs files into the root folder

**Version of emscripten/emsdk:**
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.13-git
clang version 15.0.0
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /usr/local/llvm-devel/bin

**Description**
It installs all files into root which is uncommon in UNIX. It should install executables into $PREFIX/bin and the rest of files into some folder, for example $PREFIX/lib/emscripten.

We have to use this script in the FreeBSD port to correct this problem:
```
${MKDIR} ${STAGEDIR}${PREFIX}/lib/${PORTNAME}
cd ${WRKSRC} && ${CP} -r em* cmake site src system third_party tools ${STAGEDIR}${PREFIX}/lib/${PORTNAME}/
${FIND} ${STAGEDIR}${PREFIX}/lib/${PORTNAME} \( -name "*.bat" -o -name "*.orig" \) -delete
.for s in ${PYSCRIPTS}
@(echo "#!/bin/sh"; \
echo ""; \
echo "${PYTHON_CMD} ${PREFIX}/lib/${PORTNAME}/${s}.py \"$$"@"\"" \
) > ${STAGEDIR}${PREFIX}/lib/${PORTNAME}/${s}
@${CHMOD} +x ${STAGEDIR}${PREFIX}/lib/${PORTNAME}/${s}
${RLN} ${STAGEDIR}${PREFIX}/lib/${PORTNAME}/${s} ${STAGEDIR}${PREFIX}/bin/${s}
```