sqlite_rtree_bulk_load.c: include <stdlib.h>

[schwehr]

<math.h> doesn't transitively include <stdlib.h> anymore in llvm near upstream head.

Caused by this commit:

llvm/llvm-project@584cc37

[libc++] Move std::abs into __math/abs.h (llvm/llvm-project#139586)

template <class = int> is also added to our implementations to avoid an ambiguity between the libc's version and our version when both are visible.

This avoids including <stdlib.h> in <math.h>.