libphobos: Add D runtime support code for MinGW (PR99794)

libphobos/ChangeLog:

	PR d/99794
	* libdruntime/Makefile.am (DRUNTIME_SOURCES_CONFIGURED): Add
	config/mingw/msvc.c on DRUNTIME_OS_MINGW.
	* libdruntime/Makefile.in: Regenerate.
	* libdruntime/config/mingw/msvc.c: New file.
	* libdruntime/config/mingw/switchcontext.S (fiber_switchContext): Fix
	function definition.
	* libdruntime/gcc/deh.d (__gdc_personality_seh0): Fix call to
	_GCC_specific_handler.
	* libdruntime/gcc/gthread.d (__gthread_once_t): Fix definition.
	* libdruntime/gcc/unwind/generic.d (_GCC_specific_handler): Fix
	declaration.
	* libdruntime/rt/dmain2.d (rt_loadLibrary): Remove function.
	(rt_loadLibraryW): Remove function.
	(initLibrary): Remove function.
	(rt_unloadLibrary): Remove function.