Fix fabs(3) for powerpcspe, this time for real

SPE ABI uses the soft-float ABI, which splits doubles into two words.  As such,
fabs(3) cannot work on a double directly.  It's too costly to convert the
argument pair into a single double to use efdabs, so clear the top bit of the
high word, which is the sign bit.

lib/libc/powerpcspe/gen/fabs.S

@@ -31,7 +31,8 @@ __FBSDID("$FreeBSD$");
  * double fabs(double)
  */
 ENTRY(fabs)
-	efdabs	%r3,%r3
+	/* arg is split in two words, clear sign bit only, in r3. */
+	clrlwi	%r3,%r3,1
 	blr
 END(fabs)