Julia-1.6.0 build fails because of failed links to missing dependent libraries. After installing sci-libs/umfpack and sci-libs/spqr the build completes successfully.
Thanks for your report. In order to better reproduce this issue, we ask you to post the output of emerge -pqv julia as well as attach a complete build log. See https://wiki.gentoo.org/wiki/Bugzilla/Bug_report_guide/en for details on this. Please re-open this bug, setting the status to UNCONFIRMED, once you have posted these information. Also please note, both of these packages are in RDEPEND, so they should have been pulled in when you asked to emerge julia. Therefore, the issue seems to have a different source, which we can eventually solve by your provided files.