libnspr: use the BR2_ARM_CPU_ARMV7A option

The libnspr --enable-thumb2 option was only enabled on Cortex-A8 and
Cortex-A9. This commit switches to using BR2_ARM_CPU_ARMV7A instead,
which will allow using Thumb2 for this package even with the other
Cortex-A cores.

[Peter: use BR2_ARM_CPU_HAS_THUMB2 instead]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>