This commit stops using the monotonic clock API on macOS because it is
supported only on macOS >= 10.12. Instead, it uses the fallback clock
APIs for platforms without monotonic clock support.
The use of `clock_gettime` was causing issues on Atom because, even
though we build it on a new macOS version supporting such API, some
users may run Atom on older versions of macOS. On those platforms, Atom
would crash whenever opening a file parsed with a tree-sitter grammar.