I guess it's related to glibc...
I guess that depends in what sense you mean "related". Related to your use of it, perhaps.
More directly, it is from the linux kernel; the "Attempted to kill init!" error message can be found in the source tree (see
kernel/exit.c), as can the forward trace (i.e., from a subsequent call) "not syncing" (
Unravelling that would require further poking around. I think a common cause of this in a normal context is a problem with the root filesystem. I can say that I do not think "the problem" is simply that it cannot be found or read, unless perhaps you are using an initramfs (which n.b., is probably pointless on a platform specific kernel; this is why they are not normally used on the pi at all). These are crude guesses though.
You are probably going to have to do some tedious debugging, more specifically, tracing the execution of the kernel to find out where it goes wrong. Hopefully you have two pis because being able to do this in contrast to your uClibc version might be helpful.
is there any trick to run glibc on RPi3?
Not in particular, since glibc is the basis of Raspbian and all of the other normative Pi oriented distros (as far as I aware). Prior to jessie, Debian/Raspbian actually used a slight variant, eglibc, but since that was merged back into glibc several years ago and is no longer developed, Debian 8 (jessie) marks a return to glibc proper.