Spectre probe for -mretpoline/-mindirect-branch=thunk-extern

-mretpoline is currently the clang-7.0 name for Spectre mitigation.
gcc-7.3/8.1 branches currently use -mindirect-branch=thunk-extern for this.
The default GNU linker check does not work though, you need llvm lld-7.
so add -Wl,-z,retpolineplt only where the retpoline spectre option was added.
Without linker support don't add the cflags neither, but warn.
It needs lld-7 via LDFLAGS=-fuse-ld=lld-7

There is also a new -Wl,-z,textonly option.
https://sourceware.org/ml/binutils/2017-11/msg00369.html
Probe for it also.