This helps running cross compiled programs as well as compiling programs
under Sortix with gcc. There is also support for global constructors.

Currently, cross-compiled executables uses these startup files. The current
build system continues to use start.o, which does not offer global
constructors and other useful features.

Note that these using the crtX.o files requires the crtbegin.o and crtend.o
files that ship with the cross compiler, but that should be no problem.