rts: replace ad-hoc MYTASK_USE_TLV with proper CC_SUPPORTS_TLS

This patch replaces the ad-hoc `MYTASK_USE_TLV` with the
`CC_SUPPORTS_TLS` macro. If TLS support is detected by autoconf, then
we should use that for managing `myTask` in the threaded RTS.

(cherry picked from commit f3017dd3)