Only define isnan, isnanf, __isnan and __isnanf in libc.so, not in

libc.a and libc_p.a.  In addition, define isnan in libm.a and libm_p.a,
but not in libm.so.

This makes it possible to statically link executables using both isnan
and isnanf with libc and libm.

Tested by:	kargl
MFC after:	1 week

lib/libc/gen/isnan.c

@@ -35,6 +35,7 @@
  *     binary compat until we can bump libm's major version number.
  */
 
+#ifdef PIC
 __weak_reference(__isnan, isnan);
 __weak_reference(__isnanf, isnanf);
 
@@ -55,3 +56,4 @@ __isnanf(float f)
 	u.f = f;
 	return (u.bits.exp == 255 && u.bits.man != 0);
 }
+#endif /* PIC */

lib/msun/src/s_isnan.c

@@ -30,8 +30,9 @@
 
 #include "fpmath.h"
 
-/* Provided by libc */
-#if 0
+/* Provided by libc.so */
+#ifndef PIC
+#undef isnan
 int
 isnan(double d)
 {
@@ -40,7 +41,7 @@ isnan(double d)
 	u.d = d;
 	return (u.bits.exp == 2047 && (u.bits.manl != 0 || u.bits.manh != 0));
 }
-#endif
+#endif /* !PIC */
 
 int
 __isnanf(float f)