bsdinstall: use gpt/efiboot0 label in /etc/fstab

Otherwise, boot will hang if the numbering of disks has changed since
initial install.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: brd
Differential Revision: https://reviews.freebsd.org/D35309

(cherry picked from commit 7919c76dbdd20161247d1bfb647110d87ca5ee0f)