Custom target files don't support (pre|post)_link_objects_(dll|exe)

[carlpaten]

I'm writing a target file for code that runs on the Xen hypervisor. In order to produce executable binaries, I need cargo to link with a bootloader object file. However, when producing static libraries etc., linking against this bootloader object is liable to cause all sorts of problems. I'd like to use the pre_link_objects_exe field of TargetOptions to link against the bootloader.

The fix seems like it would be simple: simply add a few key! calls in librustc_back/target/mod.rs.

I would really appreciate if this change was made, I could then publish a fully working Xen target :)

[steveklabnik]

Triage: not aware of any changes here

[petrochenkov]

Fixed in #71769.