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pc64: fix mistake fpurestore() mistake

cannot just reenable the fpu in FPactive case as we might have
been procsaved() an rescheduled on another cpu. what was i thinking...
thanks qu7uux for reproducing the problem.

--- a/sys/src/9/pc64/main.c

+++ b/sys/src/9/pc64/main.c

@@ -712,20 +712,25 @@

 fpusave(void)

 {

 	int ostate = up->fpstate;

-	if((up->fpstate & ~(FPnouser|FPkernel|FPindexm)) == FPactive)

+	if((ostate & ~(FPnouser|FPkernel|FPindexm)) == FPactive)

 		_stts();

-	up->fpstate = FPpush | (up->fpstate & ~FPillegal);

+	up->fpstate = FPpush | (ostate & ~FPillegal);

 	return ostate;

 }

 void

 fpurestore(int ostate)

 {

-	if((up->fpstate & ~(FPnouser|FPkernel|FPindexm)) == FPactive)

+	int astate = up->fpstate;

+	if((astate & ~(FPnouser|FPkernel|FPindexm)) == FPactive)

 		_stts();

-	if((ostate & FPindexm) == (up->fpstate & FPindexm)){

-		if((ostate & ~(FPnouser|FPkernel|FPindexm)) == FPactive)

+	if((astate & FPindexm) == (ostate & FPindexm)){

+		if((ostate & ~(FPnouser|FPkernel|FPindexm)) == FPactive){

+			if((astate & ~(FPpush|FPnouser|FPkernel|FPindexm)) != FPactive)

+				goto saved;

 			_clts();

+		}

 	} else {

+	saved:

 		up->fpsave = up->fpslot[ostate>>FPindexs];

 		ostate = FPinactive | (ostate & (FPillegal|FPpush|FPnouser|FPkernel|FPindexm));

 	}