readelf: Fix printing NT_FREEBSD_ARCH_TAG

Looking at lib/csu/arm/crt1_s.S, this should be a string and therefore the
restriction to 4 characters seems wrong.
Found whle updating https://reviews.llvm.org/D74393.

Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D28470

(cherry picked from commit 2a39919364b5368e026f656ff41861a3fdd56110)