https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66014
--- Comment #1 from Matt Breedlove <breedlove.matt at gmail dot com> --- This issue is still present on 5.1.0 and trunk. The issue seems to be related to libiberty's implementation of stpcpy being replaced with the GCC builtin and then causing linker errors due to that symbol not being defined when attempting to link lto-plugin. Passing in -fno-builtin-stpcpy when building libiberty has resolved this for me and allows the build to progress but I'm unsure if there is any other potential fallout from similar issues.
