uclibc/arm: doesn't build in thumb(1) mode with threads

It runs out of registers, it fails even in official form
(COMPILE_IN_THUMB_MODE=y) so just build it in ARM mode since EABI
mandates interworking.
Tested in an arm920t board.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>