Gentoo's Bugzilla –
Attachment 572432 Details for
Bug 683066cross-aarch64-unknown-linux-gnu/glibc-2.28-r6 - In file included from ../sysdeps/unix/sysv/linux/aarch64/libc-start.c:20: ../sysdeps/unix/sysv/linux/aarch64/sysdep.h:209:17: error: invalid register name for '_x0'