Enable TLS support in gcc.

Since hrev47198 we have ELF-based TLS support in Haiku. When building
gcc with haikuporter, this is detected by the configure script, but when
cross compiling gcc we need to manually enable it, as no runtime check
can be performed to detect the feature.

This should fix #10938 by avoiding the mix of TLS and non-TLS libstdc++.