eglibc/arm: doesn't build in thumb(1) mode

It runs out of registers, so build it in ARM mode, EABI mandates
interworking so this isn't an issue.
Tested in an arm920t board.

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