libthr: Preresolve selected EABI symbols on arm.

Add the ability to pre-resolve architecture-specific EABI symbols and
use it on arm for selected EABI functions. These functions can be called
with rtld bind lock write-locked, so they should be resolved in forward.

Reported by: Mark Millard <marklmi@yahoo.com>, John F Carr <jfc@mit.edu>
Reviewed by: kib, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46104