Fix GH-13789 Build failed mbstring_arginfo.h on VC++

[youkidearitai]

mbstring.stub.php uses Unicode special character.
Some environment(Japanese CP932) VC++ is fail to compile. I found solution that insert Byte Order Mark.
However, scope of impact is very big. Hence add @insert-bom block comment at .stub.php file.

[youkidearitai]

If you don't like the idea of aligning with the MSVC, please close this PR.

[youkidearitai]

Ah, This approach is wrong (/WX flag included). Therefore, close this PR. Sorry for noise.

"cl.exe" /DHAVE_MBSTRING_H=1 /D ZEND_COMPILE_DL_EXT=1 /D COMPILE_DL_MBSTRING /D MBSTRING_EXPORTS=1 -Iext/mbstring -Iext/mbstring/libmbfl -Iext/mbstring/libmbfl/mbfl /D HAVE_STRICMP /D MBFL_DLL_EXPORT=1 /DZEND_ENABLE_STATIC_TSRMLS_CACHE=1 /I "C:\build-cache\deps-master-vs16-x64\include" /DHAVE_ONIGURUMA_H=1 /DONIG_EXTERN=extern /DPHP_ONIG_BAD_KOI8_ENTRY=1 /DZEND_ENABLE_STATIC_TSRMLS_CACHE=1 /nologo /I . /I main /I Zend /I TSRM /I ext /D _WINDOWS /D WINDOWS=1 /D ZEND_WIN32=1 /D PHP_WIN32=1 /D WIN32 /D _MBCS /D _USE_MATH_DEFINES /W1 /WX /FD /wd4996 /Qspectre /guard:cf /Zc:inline /Zc:__cplusplus /d2FuncCache1 /Zc:wchar_t /MP /LD /MD /Ox /D NDebug /D NDEBUG /GF /D ZEND_DEBUG=0 /D ZTS=1 /I "C:\build-cache\deps-master-vs16-x64\include" /D FD_SETSIZE=256 /arch:AVX2 /DHAVE_ARGON2ID_HASH_RAW=1 /FoC:\obj\Release_TS\ext\mbstring\ /FpC:\obj\Release_TS\ext\mbstring\ /FRC:\obj\Release_TS\ext\mbstring\ /FdC:\obj\Release_TS\ext\mbstring\ /c ext\mbstring\mb_gpc.c ext\mbstring\mbstring.c ext\mbstring\php_unicode.c
mb_gpc.c
mbstring.c
php_unicode.c
D:\a\php-src\php-src\ext\mbstring\mbstring_arginfo.h(127): error C2220: the following warning is treated as an error (compiling source file ext\mbstring\mbstring.c)