FastISel: support no-PLT PIC calls on ELF x86_64

Add support for properly handling PIC code with no-PLT.  This equates to
`-fpic -fno-plt -O0` with the clang frontend.  External functions are
marked with nonlazybind, which must then be indirected through the GOT.
This allows code to be built without optimizations in PIC mode without
going through the PLT.  Addresses PR35653!

llvm-svn: 320776