libc: enable initial-exec (IE) as default thread-local storage model on arm

As suggested by jrtc27@ in https://reviews.freebsd.org/D42415, this
patch enables IE as default thread-local storage model in libc on arm.

Reviewed by: kib
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42445