add -Z pre-link-arg{,s} to rustc

add -Z pre-link-arg{,s} to rustc

This commit adds two unstable flags to `rustc`: `-Z pre-link-arg` and `-Z
pre-link-args`. These are the counterpart of the existing `-C link-arg{,s}`
flags and can be used to pass extra arguments at the *beginning* of the linker
invocation, before the Rust object files are passed.