toolchain: check that the thread option selection is correct

When an external toolchain is used, the user is responsible for
telling Buildroot what the configuration of the toolchain is. In order
to make sure that the selection is correct, we already perform checks
against the configuration of the uClibc in the selected external
toolchain. This commit adds the check for thread support.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>