Fix unexpected intial content in user thread stack

[iankuan]

When program initialize the second thread, we don't need to predefine apsr. It should be zero.

[jserv]

Improve Git commit messages first. It was meant "predefine apsr" but a matter of Thumb bit manipulate.

[iankuan]

I think it is more appropriate to change the commit message to "In this implementation, the second thread would be trigged by exception return. The unexpected behavior would happen on loading xpsr. It shouldn't enable carry flag in apsr at first time."

[jserv]

Change from

In this implementation, the second thread would be trigged by exception return. The unexpected behavior would happen on loading xpsr. It shouldn't enable carry flag in apsr at first time.

to

After creation of first user-level thread, the next thread is likely triggered by exception return. However, unexpected behavior occurs while the load of xPSR due to incorrect configuration on enabling carry flags in APSR at the initial stage.

[jserv]

The subject "Fix unexpected thread initialization" is not precise though.