Inspired by seeing "MIPS" go by in the commits, I've gone ahead and

implemented a (codegen) target hook for __builtin_extend_pointer.
I'm also making it return a uint64_t instead of an unsigned word;  this
comports with typical usage (i.e. the one use I know of).

I don't know if any of the existing targets requires this hook to be
set (other than x86 and x86_64, which I know do not).

llvm-svn: 97547