[gn build] Fix D126415 to only set `llvm_have_mallinfo2 = true` on linux

Otherwise mac builds are broken, e.g. http://45.33.8.238/macm1/36056/step_4.txt

../../llvm/lib/Support/Unix/Process.inc:35:10: fatal error: 'malloc.h' file not found
 #include <malloc.h>