The issue is related to kernel commit 056d28d135bca0b1d0908990338e00e9dadaf057 ("objtool: Query pkg-config for libelf location"). Reverting that change resolves the issue for me. Maybe it's too early for a bug report but that must be resolved on buildroot side sooner or later, I believe.
(In reply to Janusz Krzysztofik from comment #0) There's already a thread on the list about this issue: http://lists.busybox.net/pipermail/buildroot/2019-April/246871.html Please keep an eye on it and report there if it fixes the issue for you.
Fixed by commit 615e2edc2ae8351519448d1965f0c4eec4e793b1. Please reopen if that doesn't fix it for you.