libbpf: Hashmap.h update to fix build issues using LLVM14

A fix for the LLVM compilation error while building bpftool.
Replaces the expression:

  _Static_assert((p) == NULL || ...)

by expression:

  _Static_assert((__builtin_constant_p((p)) ? (p) == NULL : 0) || ...)

When "p" is not a constant the former is not considered to be a
constant expression by LLVM 14.

The error was introduced in the following patch-set: [1].
The error was reported here: [2].

  [1] https://lore.kernel.org/bpf/20221109142611.879983-1-eddyz87@gmail.com/
  [2] https://lore.kernel.org/all/202211110355.BcGcbZxP-lkp@intel.com/

Reported-by: kernel test robot <lkp@intel.com>
Fixes: c302378bc1 ("libbpf: Hashmap interface update to allow both long and void* keys/values")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20221110223240.1350810-1-eddyz87@gmail.com

tools/lib/bpf/hashmap.h

@@ -123,7 +123,8 @@ enum hashmap_insert_strategy {
 };
 
 #define hashmap_cast_ptr(p) ({								\
-	_Static_assert((p) == NULL || sizeof(*(p)) == sizeof(long),			\
+	_Static_assert((__builtin_constant_p((p)) ? (p) == NULL : 0) ||			\
+				sizeof(*(p)) == sizeof(long),				\
 		       #p " pointee should be a long-sized integer or a pointer");	\
 	(long *)(p);									\
 })

tools/perf/util/hashmap.h

@@ -123,7 +123,8 @@ enum hashmap_insert_strategy {
 };
 
 #define hashmap_cast_ptr(p) ({								\
-	_Static_assert((p) == NULL || sizeof(*(p)) == sizeof(long),			\
+	_Static_assert((__builtin_constant_p((p)) ? (p) == NULL : 0) ||			\
+				sizeof(*(p)) == sizeof(long),				\
 		       #p " pointee should be a long-sized integer or a pointer");	\
 	(long *)(p);									\
 })