elf: Enable TLS descriptor tests on aarch64

The aarch64 uses 'trad' for traditional tls and 'desc' for tls
descriptors, but unlike other targets it defaults to 'desc'.  The
gnutls2 configure check does not set aarch64 as an ABI that uses
TLS descriptors, which then disable somes stests.

Also rename the internal machinery fron gnu2 to tls descriptors.

Checked on aarch64-linux-gnu.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

(cherry picked from commit 3d53d18f)