crt: Align i386 stack to 16-bytes for DLL builds due to SSE

i386 DLL builds have same issue as EXE builds, entry point (DllMain or
DllEntryPoint) can use SSE and therefore stack should be aligned to
16-bytes. Use attribute force_align_arg_pointer for __DllMainCRTStartup,
like it is already for __tmainCRTStartup. Also mark it as static.

This code was copied from crtexe.c __tmainCRTStartup() function.

Signed-off-by: Martin Storsjö <martin@martin.st>