toolchain/cross-compile: Allow TLS support selection.
toolchain/cross-compile: Allow TLS support selection.

Add a user-settable configuration value indicating whether the toolchain
selected by the cross-compile variant supports thread local storage, using
the default 'n' value to preserve compatibility with existing uses.

Signed-off-by: Keith Packard <fd7f967895e9f35e58ec8a62a847a54d7fa7275f@keithp.com>

finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr