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9pc64: handle special case in fpurestore() for procexec()/procsetup()

when a process does an exec, it calls procsetup() which
unconditionally sets the sets the TS flag and fpstate=FPinit
and fpurestore() should not revert the fpstate.

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

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

@@ -721,16 +721,14 @@

 fpurestore(int ostate)

 {

 	int astate = up->fpstate;

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

-		_stts();

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

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

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

-				goto saved;

+	if(astate == (FPpush | (ostate & ~FPillegal))){

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

 			_clts();

-		}

 	} else {

-	saved:

+		if(astate == FPinit)	/* don't restore on procexec()/procsetup() */

+			return;

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

+			_stts();

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

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

 	}