Add __isnan()/__isnanf() aliases for compatibility with glibc and CUDA

Even though clang comes with a number of internal CUDA wrapper headers,
compiling sample CUDA programs will result in errors similar to:

In file included from <built-in>:1:
In file included from /usr/lib/clang/9.0.0/include/__clang_cuda_runtime_wrapper.h:204:
/usr/home/arr/cuda/var/cuda-repo-10-0-local-10.0.130-410.48/usr/local/cuda-10.0//include/crt/math_functions.hpp:2910:7: error: no matching function for call to '__isnan'
  if (__isnan(a)) {
      ^~~~~~~
/usr/lib/clang/9.0.0/include/__clang_cuda_device_functions.h:460:16: note: candidate function not viable: call to __device__ function from __host__ function
__DEVICE__ int __isnan(double __a) { return __nv_isnand(__a); }
               ^

CUDA expects __isnan() and __isnanf() declarations to be available,
which are glibc specific extensions, equivalent to the regular isnan()
and isnanf().

To provide these, define __isnan() and __isnanf() as aliases of the
already existing static inline functions __inline_isnan() and
__inline_isnanf() from math.h.

Reported by:	arrowd
PR:		241550
MFC after:	1 week

lib/libc/gen/isnan.c

@@ -43,6 +43,13 @@
  */
 
 #ifdef PIC
+/*
+ * Because math.h defines __isnan and __isnanf as aliases for compatibility with
+ * glibc and CUDA, we have to undefine them here to avoid redefinition errors.
+ */
+#undef __isnan
+#undef __isnanf
+
 __weak_reference(__isnan, isnan);
 __weak_reference(__isnanf, isnanf);
 

lib/msun/src/math.h

@@ -214,6 +214,12 @@ __inline_isnanl(__const long double __x)
 	return (__x != __x);
 }
 
+/*
+ * Define the following aliases, for compatibility with glibc and CUDA.
+ */
+#define __isnan __inline_isnan
+#define __isnanf __inline_isnanf
+
 /*
  * Version 2 of the Single UNIX Specification (UNIX98) defined isnan() and
  * isinf() as functions taking double.  C99, and the subsequent POSIX revisions

lib/msun/src/s_isnan.c

@@ -45,6 +45,12 @@ isnan(double d)
 }
 #endif /* !PIC */
 
+/*
+ * Because math.h defines __isnanf as an alias for compatibility with glibc and
+ * CUDA, we have to undefine it here to avoid redefinition errors.
+ */
+#undef __isnanf
+
 int
 __isnanf(float f)
 {