Initialize thread regs with 0xDEADBEEF.

Also initialize the float regs to NaN.
Fill registers with 0xDEADBEEF after most syscalls.

Some, like sceMpegRingbufferPut(), we don't, since... it's complicated.
But most we do, like a real PSP does.
Add a setting to skip 0xDEADBEEF reg filling.

For debugging purposes.  Can't think of a better way.
Merge pull request #4369 from unknownbrackets/deadbeef

Fill regs with 0xDEADBEEF after syscalls...