5l, 6l, 8l: correct ELFRESERVE diagnostic

If the length of the interpreter string
pushes us over the ELFRESERVE limit, the
resulting error message will be comical.

I was doing some ELF tinkering with a
modified version of 8l when I hit this.

To be clear, the stock linkers wouldn't
hit this without adding about forty more
section headers.  We're safe for now. ;)

Also, remove a redundant call to cflush.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5268044

src/cmd/5l/asm.c

@@ -632,8 +632,8 @@ asmb(void)
 		a += elfwritehdr();
 		a += elfwritephdrs();
 		a += elfwriteshdrs();
-		cflush();
-		if(a+elfwriteinterp() > ELFRESERVE)	
+		a += elfwriteinterp();
+		if(a > ELFRESERVE)	
 			diag("ELFRESERVE too small: %d > %d", a, ELFRESERVE);
 		break;
 	}

src/cmd/6l/asm.c

@@ -1095,8 +1095,8 @@ asmb(void)
 		a += elfwritehdr();
 		a += elfwritephdrs();
 		a += elfwriteshdrs();
-		cflush();
-		if(a+elfwriteinterp() > ELFRESERVE)	
+		a += elfwriteinterp();
+		if(a > ELFRESERVE)	
 			diag("ELFRESERVE too small: %d > %d", a, ELFRESERVE);
 		break;
 	case Hwindows:

src/cmd/8l/asm.c

@@ -1160,8 +1160,8 @@ asmb(void)
 		a += elfwritehdr();
 		a += elfwritephdrs();
 		a += elfwriteshdrs();
-		cflush();
-		if(a+elfwriteinterp() > ELFRESERVE)	
+		a += elfwriteinterp();
+		if(a > ELFRESERVE)	
 			diag("ELFRESERVE too small: %d > %d", a, ELFRESERVE);
 		break;