target/Config.in.arch: fix BR2_GCC_TARGET_ARCH for arm926t

Closes #367.

arm926t was wrongly listed as armv4t, and not armv5te - leading to
the following error while compiling the kernel (2.6.28):

  CC      arch/arm/kernel/traps.o
{standard input}: Assembler messages:
{standard input}:1094: Error: selected processor does not support `pld [r6,#0]'

Signed-off-by: Alex Dobrynin <alex_dobrynin@hotmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>