sim: pru: Fix emulation of carry bit

sim: pru: Fix emulation of carry bit

The PRU architecture documentation [1] was used for the initial GNU
simulator implementation.  But recently [2] TI confirmed the carry
behaviour was wrongly documented.  In reality, the PRU carry behaves
like the carry in ARM processors.

This patch fixes simulator to align with latest recommendations from TI.

The new carry.s test was also validated to pass on real hardware -
a BeaglePlay board [3].  That test is a bit long because TI still
has not released official updates for the PRU documents.  And I wanted
to ensure simulator handles all edge cases exactly as the real hardware
does.

[1] https://www.ti.com/lit/pdf/spruij2
[2] https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1244359/sk-am64b-am64x-pru-assembler-how-works-this-bloody-carry
[3] https://www.beagleboard.org/boards/beagleplay

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>