os_test: arm64: disable C++ and TLS tests depending on compiler

In 64-bit mode, the Thead Local Storage tests currently pass only with
Clang, or GCC 8 or later. Earlier GCC versions (6.2, 7.3) generate an
unsupported relocation type:

 * regression_1006 Test Basic OS features
 E/LD:  e64_relocate:538 Unknown relocation type 1031
 E/TC:?? 0 init_with_ldelf:232 ldelf failed with res: 0xffff0005
 [...]
   regression_1006 FAILED

TLS is not currently useful in itself, since OP-TEE doesn't have multi-
threaded TAs. It was implemented primarily to support C++ because the
GNU C++ runtime depends on it. To avoid failures in the regression
tests of people who upgrade their OP-TEE environment and who possibly
use older toolchains, the TLS tests should be ignored unless a "known
good" compiler is used.

Similarly, since the C++ tests depend on TLS, they need to be disabled
too with Aarch64 GCC < 8 (note they are already disabled with Clang).

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Tested-by: Jerome Forissier <jerome@forissier.org> (QEMU/QEMUv8, GCC 8.3/GCC 6.2/Clang 10)
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>