Make Cortex-M0 set/clear interrupt flag from ISR functions nestable.

Make Cortex-M0 set/clear interrupt flag from ISR functions nestable.
Don't reset the stack location when starting the scheduler in Cortex-M0 ports as the vector offset register is not implemented and XMC1000 devices have their application vector address somewhere other than 0x00.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2015 1d2547de-c912-0410-9cb9-b8ca96c0e9e2