Toolchains: remove crtbegin on xwindows

crtbegin is not really a proper windows support thing.  This was
duplicated when the toolchain was initially built.  If the injection of
crtbegin is needed, it can be done via the `/include` directive.
Furthermore, since `-fPIC` doesnt make sense on PE/COFF, crtbegin and
crtbeginS dont really need to be different.

llvm-svn: 299800