build: Remove the cmake check for malloc.h.

As far as I can tell, malloc.h is only being used here to provide
a definition of mallinfo (malloc itself is declared in stdlib.h via
cstdlib). We already have a macro for whether mallinfo is available,
so switch to using that instead.

Differential Revision: https://reviews.llvm.org/D57807

llvm-svn: 353329