rtld/arm64: Remove checks for undefined symbols when processing TPREL64

lld emits several GOT relocations referencing the null sumbol in libc.so
when compiled with -ftls-model=initial-exec. This symbol is specified
to be undefined.

We generally do not handle dynamic TLS relocations against weak,
undefined symbols, so avoid printing a warning here. This makes it
possible to compile libc.so using the initial-exec TLS model on arm64.

Reviewed by: jrtc27, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31069