csu: add code to call constructors to be used by clang
. clang-linked binaries were not calling global constructors, as the
code to do so wasn't in csu/ and linked
. it does work for gcc as it uses its self-supplied crt{begin,end} code
. this commit copies netbsd's crt{begin,end}.S, which contains
constructor/destructor calling code, called from .init and .fini
sections already accumulated by the linker. the _init function was already
called by the C startup code before calling main.
. based on work by Antoine Leca