Rename gdbarch_tdep template function to gdbarch_tdep_cast for g++ 4.8

commit 08106042 "gdb: move the type cast into gdbarch_tdep" created
a template function with the same name as a struct gdbarch_tdep. This
confused g++ 4.8 believing the struct gdbarch_tdep was an incomplete
type. Fix this by renaming the template function to gdbarch_tdep_cast.