TL;DR
Rebase your CL if you get PreCQ failures for x86-alex.
Details
We recently removed a number of EOL'd boards from various parts of the build system. Because prebuilt binaries for Chrome are no longer available, they are not expected to build for PreCQ build configs. We removed these boards from the PreCQ configs, BUT CLs uploaded before we removed them still try to use the old list of builders.
If you upload a revised (even just rebased) version of the same CL, it will re-select the builders to test against, get an updated list, and be tested normally.
Sorry for the inconvenience.